memgraph/src/query/plan/operator.cpp

#include "query/plan/operator.hpp"

#include <algorithm>
#include <future>
#include <limits>
#include <queue>
#include <random>
#include <string>
#include <type_traits>
#include <utility>

#include "boost/archive/binary_iarchive.hpp"
#include "boost/archive/binary_oarchive.hpp"
#include "boost/serialization/export.hpp"
#include "glog/logging.h"

#include "database/graph_db_accessor.hpp"
#include "distributed/remote_pull_rpc_clients.hpp"
#include "distributed/remote_updates_rpc_clients.hpp"
#include "distributed/remote_updates_rpc_server.hpp"
#include "query/context.hpp"
#include "query/exceptions.hpp"
#include "query/frontend/ast/ast.hpp"
#include "query/frontend/semantic/symbol_table.hpp"
#include "query/interpret/eval.hpp"
#include "query/path.hpp"
#include "utils/algorithm.hpp"
#include "utils/exceptions.hpp"

DEFINE_HIDDEN_int32(remote_pull_sleep_micros, 10,
                    "Sleep between remote result pulling in microseconds");

// macro for the default implementation of LogicalOperator::Accept
// that accepts the visitor and visits it's input_ operator
#define ACCEPT_WITH_INPUT(class_name)                                    \
  bool class_name::Accept(HierarchicalLogicalOperatorVisitor &visitor) { \
    if (visitor.PreVisit(*this)) {                                       \
      input_->Accept(visitor);                                           \
    }                                                                    \
    return visitor.PostVisit(*this);                                     \
  }

#define WITHOUT_SINGLE_INPUT(class_name)                                 \
  bool class_name::HasSingleInput() const { return false; }              \
  std::shared_ptr<LogicalOperator> class_name::input() const {           \
    LOG(FATAL) << "Operator " << #class_name << " has no single input!"; \
  }                                                                      \
  void class_name::set_input(std::shared_ptr<LogicalOperator>) {         \
    LOG(FATAL) << "Operator " << #class_name << " has no single input!"; \
  }

namespace query::plan {

namespace {

// Sets a property on a record accessor from a TypedValue. In cases when the
// TypedValue cannot be converted to PropertyValue,
// QueryRuntimeException is raised.
template <class TRecordAccessor>
void PropsSetChecked(TRecordAccessor &record, storage::Property key,
                     TypedValue value) {
  try {
    record.PropsSet(key, value);
  } catch (const TypedValueException &) {
    throw QueryRuntimeException("'{}' cannot be used as a property value.",
                                value.type());
  } catch (const RecordDeletedError &) {
    throw QueryRuntimeException(
        "Trying to set properties on a deleted graph element.");
  }
}

// Checks if the given value of the symbol has the expected type. If not, raises
// QueryRuntimeException.
void ExpectType(Symbol symbol, TypedValue value, TypedValue::Type expected) {
  if (value.type() != expected)
    throw QueryRuntimeException("Expected a {} for '{}', but got {}.", expected,
                                symbol.name(), value.type());
}

// Returns boolean result of evaluating filter expression. Null is treated as
// false. Other non boolean values raise a QueryRuntimeException.
bool EvaluateFilter(ExpressionEvaluator &evaluator, Expression *filter) {
  TypedValue result = filter->Accept(evaluator);
  // Null is treated like false.
  if (result.IsNull()) return false;
  if (result.type() != TypedValue::Type::Bool)
    throw QueryRuntimeException(
        "Filter expression must be a bool or null, but got {}.", result.type());
  return result.Value<bool>();
}

}  // namespace

bool Once::OnceCursor::Pull(Frame &, Context &) {
  if (!did_pull_) {
    did_pull_ = true;
    return true;
  }
  return false;
}

std::unique_ptr<Cursor> Once::MakeCursor(database::GraphDbAccessor &) const {
  return std::make_unique<OnceCursor>();
}

WITHOUT_SINGLE_INPUT(Once);

void Once::OnceCursor::Reset() { did_pull_ = false; }

CreateNode::CreateNode(const std::shared_ptr<LogicalOperator> &input,
                       NodeAtom *node_atom, bool on_random_worker)
    : input_(input ? input : std::make_shared<Once>()),
      node_atom_(node_atom),
      on_random_worker_(on_random_worker) {}

namespace {

// Returns a random worker id. Worker ID is obtained from the Db.
int RandomWorkerId(database::GraphDb &db) {
  thread_local std::mt19937 gen_{std::random_device{}()};
  thread_local std::uniform_int_distribution<int> rand_;

  auto worker_ids = db.GetWorkerIds();
  return worker_ids[rand_(gen_) % worker_ids.size()];
}

// Creates a vertex on this GraphDb. Returns a reference to vertex placed on the
// frame.
VertexAccessor &CreateLocalVertex(NodeAtom *node_atom, Frame &frame,
                                  Context &context) {
  auto &dba = context.db_accessor_;
  auto new_node = dba.InsertVertex();
  for (auto label : node_atom->labels_) new_node.add_label(label);

  // Evaluator should use the latest accessors, as modified in this query, when
  // setting properties on new nodes.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, dba, GraphView::NEW);
  for (auto &kv : node_atom->properties_)
    PropsSetChecked(new_node, kv.first.second, kv.second->Accept(evaluator));
  frame[context.symbol_table_.at(*node_atom->identifier_)] = new_node;
  return frame[context.symbol_table_.at(*node_atom->identifier_)].ValueVertex();
}

// Creates a vertex on the GraphDb with the given worker_id. Can be this worker.
VertexAccessor &CreateVertexOnWorker(int worker_id, NodeAtom *node_atom,
                                     Frame &frame, Context &context) {
  auto &dba = context.db_accessor_;

  if (worker_id == dba.db().WorkerId())
    return CreateLocalVertex(node_atom, frame, context);

  std::unordered_map<storage::Property, query::TypedValue> properties;

  // Evaluator should use the latest accessors, as modified in this query, when
  // setting properties on new nodes.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, dba, GraphView::NEW);
  for (auto &kv : node_atom->properties_) {
    auto value = kv.second->Accept(evaluator);
    if (!value.IsPropertyValue()) {
      throw QueryRuntimeException("'{}' cannot be used as a property value.",
                                  value.type());
    }
    properties.emplace(kv.first.second, std::move(value));
  }

  auto new_node =
      dba.InsertVertexIntoRemote(worker_id, node_atom->labels_, properties);
  frame[context.symbol_table_.at(*node_atom->identifier_)] = new_node;
  return frame[context.symbol_table_.at(*node_atom->identifier_)].ValueVertex();
}
}  // namespace

ACCEPT_WITH_INPUT(CreateNode)

std::unique_ptr<Cursor> CreateNode::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<CreateNodeCursor>(*this, db);
}

std::vector<Symbol> CreateNode::ModifiedSymbols(
    const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(table.at(*node_atom_->identifier_));
  return symbols;
}

CreateNode::CreateNodeCursor::CreateNodeCursor(const CreateNode &self,
                                               database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool CreateNode::CreateNodeCursor::Pull(Frame &frame, Context &context) {
  if (input_cursor_->Pull(frame, context)) {
    if (self_.on_random_worker_) {
      CreateVertexOnWorker(RandomWorkerId(db_.db()), self_.node_atom_, frame,
                           context);
    } else {
      CreateLocalVertex(self_.node_atom_, frame, context);
    }
    return true;
  }
  return false;
}

void CreateNode::CreateNodeCursor::Reset() { input_cursor_->Reset(); }

CreateExpand::CreateExpand(NodeAtom *node_atom, EdgeAtom *edge_atom,
                           const std::shared_ptr<LogicalOperator> &input,
                           Symbol input_symbol, bool existing_node)
    : node_atom_(node_atom),
      edge_atom_(edge_atom),
      input_(input ? input : std::make_shared<Once>()),
      input_symbol_(input_symbol),
      existing_node_(existing_node) {}

ACCEPT_WITH_INPUT(CreateExpand)

std::unique_ptr<Cursor> CreateExpand::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<CreateExpandCursor>(*this, db);
}

std::vector<Symbol> CreateExpand::ModifiedSymbols(
    const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(table.at(*node_atom_->identifier_));
  symbols.emplace_back(table.at(*edge_atom_->identifier_));
  return symbols;
}

CreateExpand::CreateExpandCursor::CreateExpandCursor(
    const CreateExpand &self, database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool CreateExpand::CreateExpandCursor::Pull(Frame &frame, Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  // get the origin vertex
  TypedValue &vertex_value = frame[self_.input_symbol_];
  ExpectType(self_.input_symbol_, vertex_value, TypedValue::Type::Vertex);
  auto &v1 = vertex_value.Value<VertexAccessor>();

  // Similarly to CreateNode, newly created edges and nodes should use the
  // latest accesors.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::NEW);
  // E.g. we pickup new properties: `CREATE (n {p: 42}) -[:r {ep: n.p}]-> ()`
  v1.SwitchNew();

  // get the destination vertex (possibly an existing node)
  auto &v2 = OtherVertex(v1.GlobalAddress().worker_id(), frame, context);
  v2.SwitchNew();

  // create an edge between the two nodes
  switch (self_.edge_atom_->direction_) {
    case EdgeAtom::Direction::IN:
      CreateEdge(v2, v1, frame, context.symbol_table_, evaluator);
      break;
    case EdgeAtom::Direction::OUT:
      CreateEdge(v1, v2, frame, context.symbol_table_, evaluator);
      break;
    case EdgeAtom::Direction::BOTH:
      // in the case of an undirected CreateExpand we choose an arbitrary
      // direction. this is used in the MERGE clause
      // it is not allowed in the CREATE clause, and the semantic
      // checker needs to ensure it doesn't reach this point
      CreateEdge(v1, v2, frame, context.symbol_table_, evaluator);
  }

  return true;
}

void CreateExpand::CreateExpandCursor::Reset() { input_cursor_->Reset(); }

VertexAccessor &CreateExpand::CreateExpandCursor::OtherVertex(
    int worker_id, Frame &frame, Context &context) {
  if (self_.existing_node_) {
    const auto &dest_node_symbol =
        context.symbol_table_.at(*self_.node_atom_->identifier_);
    TypedValue &dest_node_value = frame[dest_node_symbol];
    ExpectType(dest_node_symbol, dest_node_value, TypedValue::Type::Vertex);
    return dest_node_value.Value<VertexAccessor>();
  } else {
    return CreateVertexOnWorker(worker_id, self_.node_atom_, frame, context);
  }
}

void CreateExpand::CreateExpandCursor::CreateEdge(
    VertexAccessor &from, VertexAccessor &to, Frame &frame,
    const SymbolTable &symbol_table, ExpressionEvaluator &evaluator) {
  EdgeAccessor edge =
      db_.InsertEdge(from, to, self_.edge_atom_->edge_types_[0]);
  for (auto kv : self_.edge_atom_->properties_)
    PropsSetChecked(edge, kv.first.second, kv.second->Accept(evaluator));
  frame[symbol_table.at(*self_.edge_atom_->identifier_)] = edge;
}

template <class TVerticesFun>
class ScanAllCursor : public Cursor {
 public:
  explicit ScanAllCursor(Symbol output_symbol,
                         std::unique_ptr<Cursor> &&input_cursor,
                         TVerticesFun &&get_vertices,
                         database::GraphDbAccessor &db)
      : output_symbol_(output_symbol),
        input_cursor_(std::move(input_cursor)),
        get_vertices_(std::move(get_vertices)),
        db_(db) {}

  bool Pull(Frame &frame, Context &context) override {
    if (db_.should_abort()) throw HintedAbortError();
    if (!vertices_ || vertices_it_.value() == vertices_.value().end()) {
      if (!input_cursor_->Pull(frame, context)) return false;
      // We need a getter function, because in case of exhausting a lazy
      // iterable, we cannot simply reset it by calling begin().
      vertices_.emplace(get_vertices_(frame, context));
      vertices_it_.emplace(vertices_.value().begin());
    }

    // if vertices_ is empty then we are done even though we have just
    // reinitialized vertices_it_
    if (vertices_it_.value() == vertices_.value().end()) return false;

    frame[output_symbol_] = *vertices_it_.value()++;
    return true;
  }

  void Reset() override {
    input_cursor_->Reset();
    vertices_ = std::experimental::nullopt;
    vertices_it_ = std::experimental::nullopt;
  }

 private:
  const Symbol output_symbol_;
  const std::unique_ptr<Cursor> input_cursor_;
  TVerticesFun get_vertices_;
  std::experimental::optional<
      typename std::result_of<TVerticesFun(Frame &, Context &)>::type>
      vertices_;
  std::experimental::optional<decltype(vertices_.value().begin())> vertices_it_;
  database::GraphDbAccessor &db_;
};

ScanAll::ScanAll(const std::shared_ptr<LogicalOperator> &input,
                 Symbol output_symbol, GraphView graph_view)
    : input_(input ? input : std::make_shared<Once>()),
      output_symbol_(output_symbol),
      graph_view_(graph_view) {
  CHECK(graph_view != GraphView::AS_IS)
      << "ScanAll must have explicitly defined GraphView";
}

ACCEPT_WITH_INPUT(ScanAll)

std::unique_ptr<Cursor> ScanAll::MakeCursor(
    database::GraphDbAccessor &db) const {
  auto vertices = [this, &db](Frame &, Context &) {
    return db.Vertices(graph_view_ == GraphView::NEW);
  };
  return std::make_unique<ScanAllCursor<decltype(vertices)>>(
      output_symbol_, input_->MakeCursor(db), std::move(vertices), db);
}

std::vector<Symbol> ScanAll::ModifiedSymbols(const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(output_symbol_);
  return symbols;
}

ScanAllByLabel::ScanAllByLabel(const std::shared_ptr<LogicalOperator> &input,
                               Symbol output_symbol, storage::Label label,
                               GraphView graph_view)
    : ScanAll(input, output_symbol, graph_view), label_(label) {}

ACCEPT_WITH_INPUT(ScanAllByLabel)

std::unique_ptr<Cursor> ScanAllByLabel::MakeCursor(
    database::GraphDbAccessor &db) const {
  auto vertices = [this, &db](Frame &, Context &) {
    return db.Vertices(label_, graph_view_ == GraphView::NEW);
  };
  return std::make_unique<ScanAllCursor<decltype(vertices)>>(
      output_symbol_, input_->MakeCursor(db), std::move(vertices), db);
}

ScanAllByLabelPropertyRange::ScanAllByLabelPropertyRange(
    const std::shared_ptr<LogicalOperator> &input, Symbol output_symbol,
    storage::Label label, storage::Property property,
    std::experimental::optional<Bound> lower_bound,
    std::experimental::optional<Bound> upper_bound, GraphView graph_view)
    : ScanAll(input, output_symbol, graph_view),
      label_(label),
      property_(property),
      lower_bound_(lower_bound),
      upper_bound_(upper_bound) {
  DCHECK(lower_bound_ || upper_bound_) << "Only one bound can be left out";
}

ACCEPT_WITH_INPUT(ScanAllByLabelPropertyRange)

std::unique_ptr<Cursor> ScanAllByLabelPropertyRange::MakeCursor(
    database::GraphDbAccessor &db) const {
  auto vertices = [this, &db](Frame &frame, Context &context) {
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db, graph_view_);
    auto convert = [&evaluator](const auto &bound)
        -> std::experimental::optional<utils::Bound<PropertyValue>> {
          if (!bound) return std::experimental::nullopt;
          return std::experimental::make_optional(utils::Bound<PropertyValue>(
              bound.value().value()->Accept(evaluator), bound.value().type()));
        };
    return db.Vertices(label_, property_, convert(lower_bound()),
                       convert(upper_bound()), graph_view_ == GraphView::NEW);
  };
  return std::make_unique<ScanAllCursor<decltype(vertices)>>(
      output_symbol_, input_->MakeCursor(db), std::move(vertices), db);
}

ScanAllByLabelPropertyValue::ScanAllByLabelPropertyValue(
    const std::shared_ptr<LogicalOperator> &input, Symbol output_symbol,
    storage::Label label, storage::Property property, Expression *expression,
    GraphView graph_view)
    : ScanAll(input, output_symbol, graph_view),
      label_(label),
      property_(property),
      expression_(expression) {
  DCHECK(expression) << "Expression is not optional.";
}

ACCEPT_WITH_INPUT(ScanAllByLabelPropertyValue)

class ScanAllByLabelPropertyValueCursor : public Cursor {
 public:
  ScanAllByLabelPropertyValueCursor(const ScanAllByLabelPropertyValue &self,
                                    database::GraphDbAccessor &db)
      : self_(self), db_(db), input_cursor_(self_.input()->MakeCursor(db_)) {}

  bool Pull(Frame &frame, Context &context) override {
    if (db_.should_abort()) throw HintedAbortError();
    if (!vertices_ || vertices_it_.value() == vertices_.value().end()) {
      if (!input_cursor_->Pull(frame, context)) return false;
      ExpressionEvaluator evaluator(frame, context.parameters_,
                                    context.symbol_table_, db_,
                                    self_.graph_view());
      TypedValue value = self_.expression()->Accept(evaluator);
      if (value.IsNull()) return Pull(frame, context);
      try {
        vertices_.emplace(db_.Vertices(self_.label(), self_.property(), value,
                                       self_.graph_view() == GraphView::NEW));
      } catch (const TypedValueException &) {
        throw QueryRuntimeException("'{}' cannot be used as a property value.",
                                    value.type());
      }
      vertices_it_.emplace(vertices_.value().begin());
    }

    // if vertices_ is empty then we are done even though we have just
    // reinitialized vertices_it_
    if (vertices_it_.value() == vertices_.value().end()) return false;

    frame[self_.output_symbol()] = *vertices_it_.value()++;
    return true;
  }

  void Reset() override {
    input_cursor_->Reset();
    vertices_ = std::experimental::nullopt;
    vertices_it_ = std::experimental::nullopt;
  }

 private:
  const ScanAllByLabelPropertyValue &self_;
  database::GraphDbAccessor &db_;
  const std::unique_ptr<Cursor> input_cursor_;
  std::experimental::optional<decltype(
      db_.Vertices(self_.label(), self_.property(), TypedValue::Null, false))>
      vertices_;
  std::experimental::optional<decltype(vertices_.value().begin())> vertices_it_;
};

std::unique_ptr<Cursor> ScanAllByLabelPropertyValue::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<ScanAllByLabelPropertyValueCursor>(*this, db);
}

ExpandCommon::ExpandCommon(Symbol node_symbol, Symbol edge_symbol,
                           EdgeAtom::Direction direction,
                           const std::vector<storage::EdgeType> &edge_types,
                           const std::shared_ptr<LogicalOperator> &input,
                           Symbol input_symbol, bool existing_node,
                           GraphView graph_view)
    : node_symbol_(node_symbol),
      edge_symbol_(edge_symbol),
      direction_(direction),
      edge_types_(edge_types),
      input_(input ? input : std::make_shared<Once>()),
      input_symbol_(input_symbol),
      existing_node_(existing_node),
      graph_view_(graph_view) {}

bool ExpandCommon::HandleExistingNode(const VertexAccessor &new_node,
                                      Frame &frame) const {
  if (existing_node_) {
    TypedValue &old_node_value = frame[node_symbol_];
    // old_node_value may be Null when using optional matching
    if (old_node_value.IsNull()) return false;
    ExpectType(node_symbol_, old_node_value, TypedValue::Type::Vertex);
    return old_node_value.Value<VertexAccessor>() == new_node;
  } else {
    frame[node_symbol_] = new_node;
    return true;
  }
}

ACCEPT_WITH_INPUT(Expand)

std::unique_ptr<Cursor> Expand::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<ExpandCursor>(*this, db);
}

std::vector<Symbol> Expand::ModifiedSymbols(const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(node_symbol());
  symbols.emplace_back(edge_symbol());
  return symbols;
}

Expand::ExpandCursor::ExpandCursor(const Expand &self,
                                   database::GraphDbAccessor &db)
    : self_(self), input_cursor_(self.input_->MakeCursor(db)), db_(db) {}

bool Expand::ExpandCursor::Pull(Frame &frame, Context &context) {
  // A helper function for expanding a node from an edge.
  auto pull_node = [this, &frame](const EdgeAccessor &new_edge,
                                  EdgeAtom::Direction direction) {
    if (self_.existing_node_) return;
    switch (direction) {
      case EdgeAtom::Direction::IN:
        frame[self_.node_symbol_] = new_edge.from();
        break;
      case EdgeAtom::Direction::OUT:
        frame[self_.node_symbol_] = new_edge.to();
        break;
      case EdgeAtom::Direction::BOTH:
        LOG(FATAL) << "Must indicate exact expansion direction here";
    }
  };

  auto push_future_edge = [this, &frame](auto edge, auto direction) {
    auto edge_to = std::async(std::launch::async, [edge, direction]() {
      if (direction == EdgeAtom::Direction::IN)
        return std::make_pair(edge, edge.from());
      if (direction == EdgeAtom::Direction::OUT)
        return std::make_pair(edge, edge.to());
      LOG(FATAL) << "Must indicate exact expansion direction here";
    });
    future_expands_.emplace_back(
        FutureExpand{std::move(edge_to), frame.elems()});
  };

  auto find_ready_future = [this]() {
    return std::find_if(future_expands_.begin(), future_expands_.end(),
                        [](const auto &future) {
                          return utils::IsFutureReady(future.edge_to);
                        });
  };

  auto put_future_edge_on_frame = [this, &frame](auto &future) {
    auto edge_to = future.edge_to.get();
    frame.elems() = future.frame_elems;
    frame[self_.edge_symbol_] = edge_to.first;
    frame[self_.node_symbol_] = edge_to.second;
  };

  while (true) {
    if (db_.should_abort()) throw HintedAbortError();
    // Try to get any remote edges we may have available first. If we yielded
    // all of the local edges first, we may accumulate large amounts of future
    // edges.
    {
      auto future_it = find_ready_future();
      if (future_it != future_expands_.end()) {
        // Backup the current frame (if we haven't done so already) before
        // putting the future edge.
        if (last_frame_.empty()) last_frame_ = frame.elems();
        put_future_edge_on_frame(*future_it);
        // Erase the future and return true to yield the result.
        future_expands_.erase(future_it);
        return true;
      }
    }
    // In case we have replaced the frame with the one for a future edge,
    // restore it.
    if (!last_frame_.empty()) {
      frame.elems() = last_frame_;
      last_frame_.clear();
    }
    // attempt to get a value from the incoming edges
    if (in_edges_ && *in_edges_it_ != in_edges_->end()) {
      auto edge = *(*in_edges_it_)++;
      if (edge.address().is_local() || self_.existing_node_) {
        frame[self_.edge_symbol_] = edge;
        pull_node(edge, EdgeAtom::Direction::IN);
        return true;
      } else {
        push_future_edge(edge, EdgeAtom::Direction::IN);
        continue;
      }
    }

    // attempt to get a value from the outgoing edges
    if (out_edges_ && *out_edges_it_ != out_edges_->end()) {
      auto edge = *(*out_edges_it_)++;
      // when expanding in EdgeAtom::Direction::BOTH directions
      // we should do only one expansion for cycles, and it was
      // already done in the block above
      if (self_.direction_ == EdgeAtom::Direction::BOTH && edge.is_cycle())
        continue;
      if (edge.address().is_local() || self_.existing_node_) {
        frame[self_.edge_symbol_] = edge;
        pull_node(edge, EdgeAtom::Direction::OUT);
        return true;
      } else {
        push_future_edge(edge, EdgeAtom::Direction::OUT);
        continue;
      }
    }

    // if we are here, either the edges have not been initialized,
    // or they have been exhausted. attempt to initialize the edges,
    // if the input is exhausted
    if (!InitEdges(frame, context)) {
      // We are done with local and remote edges so return false.
      if (future_expands_.empty()) return false;
      // We still need to yield remote edges.
      auto future_it = find_ready_future();
      if (future_it != future_expands_.end()) {
        put_future_edge_on_frame(*future_it);
        // Erase the future and return true to yield the result.
        future_expands_.erase(future_it);
        return true;
      }
      // We are still waiting for future edges, so sleep and fallthrough to
      // continue the loop.
      std::this_thread::sleep_for(
          std::chrono::microseconds(FLAGS_remote_pull_sleep_micros));
    }

    // we have re-initialized the edges, continue with the loop
  }
}

void Expand::ExpandCursor::Reset() {
  input_cursor_->Reset();
  in_edges_ = std::experimental::nullopt;
  in_edges_it_ = std::experimental::nullopt;
  out_edges_ = std::experimental::nullopt;
  out_edges_it_ = std::experimental::nullopt;
  future_expands_.clear();
  last_frame_.clear();
}

namespace {
// Switch the given [Vertex/Edge]Accessor to the desired state.
template <typename TAccessor>
void SwitchAccessor(TAccessor &accessor, GraphView graph_view) {
  switch (graph_view) {
    case GraphView::NEW:
      accessor.SwitchNew();
      break;
    case GraphView::OLD:
      accessor.SwitchOld();
      break;
    case GraphView::AS_IS:
      break;
  }
}
}  // namespace

bool Expand::ExpandCursor::InitEdges(Frame &frame, Context &context) {
  // Input Vertex could be null if it is created by a failed optional match. In
  // those cases we skip that input pull and continue with the next.
  while (true) {
    if (!input_cursor_->Pull(frame, context)) return false;
    TypedValue &vertex_value = frame[self_.input_symbol_];

    // Null check due to possible failed optional match.
    if (vertex_value.IsNull()) continue;

    ExpectType(self_.input_symbol_, vertex_value, TypedValue::Type::Vertex);
    auto &vertex = vertex_value.Value<VertexAccessor>();
    SwitchAccessor(vertex, self_.graph_view_);

    auto direction = self_.direction_;
    if (direction == EdgeAtom::Direction::IN ||
        direction == EdgeAtom::Direction::BOTH) {
      if (self_.existing_node_) {
        TypedValue &existing_node = frame[self_.node_symbol_];
        // old_node_value may be Null when using optional matching
        if (!existing_node.IsNull()) {
          ExpectType(self_.node_symbol_, existing_node,
                     TypedValue::Type::Vertex);
          in_edges_.emplace(
              vertex.in(existing_node.ValueVertex(), &self_.edge_types()));
        }
      } else {
        in_edges_.emplace(vertex.in(&self_.edge_types()));
      }
      in_edges_it_.emplace(in_edges_->begin());
    }

    if (direction == EdgeAtom::Direction::OUT ||
        direction == EdgeAtom::Direction::BOTH) {
      if (self_.existing_node_) {
        TypedValue &existing_node = frame[self_.node_symbol_];
        // old_node_value may be Null when using optional matching
        if (!existing_node.IsNull()) {
          ExpectType(self_.node_symbol_, existing_node,
                     TypedValue::Type::Vertex);
          out_edges_.emplace(
              vertex.out(existing_node.ValueVertex(), &self_.edge_types()));
        }
      } else {
        out_edges_.emplace(vertex.out(&self_.edge_types()));
      }
      out_edges_it_.emplace(out_edges_->begin());
    }

    return true;
  }
}

ExpandVariable::ExpandVariable(
    Symbol node_symbol, Symbol edge_symbol, EdgeAtom::Type type,
    EdgeAtom::Direction direction,
    const std::vector<storage::EdgeType> &edge_types, bool is_reverse,
    Expression *lower_bound, Expression *upper_bound,
    const std::shared_ptr<LogicalOperator> &input, Symbol input_symbol,
    bool existing_node, Lambda filter_lambda,
    std::experimental::optional<Lambda> weight_lambda,
    std::experimental::optional<Symbol> total_weight, GraphView graph_view)
    : ExpandCommon(node_symbol, edge_symbol, direction, edge_types, input,
                   input_symbol, existing_node, graph_view),
      type_(type),
      is_reverse_(is_reverse),
      lower_bound_(lower_bound),
      upper_bound_(upper_bound),
      filter_lambda_(filter_lambda),
      weight_lambda_(weight_lambda),
      total_weight_(total_weight) {
  DCHECK(type_ == EdgeAtom::Type::DEPTH_FIRST ||
         type_ == EdgeAtom::Type::BREADTH_FIRST ||
         type_ == EdgeAtom::Type::WEIGHTED_SHORTEST_PATH)
      << "ExpandVariable can only be used with breadth first, depth first or "
         "weighted shortest path type";
  DCHECK(!(type_ == EdgeAtom::Type::BREADTH_FIRST && is_reverse))
      << "Breadth first expansion can't be reversed";
}

ACCEPT_WITH_INPUT(ExpandVariable)

std::vector<Symbol> ExpandVariable::ModifiedSymbols(
    const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(node_symbol());
  symbols.emplace_back(edge_symbol());
  return symbols;
}

namespace {
/**
 * Helper function that returns an iterable over
 * <EdgeAtom::Direction, EdgeAccessor> pairs
 * for the given params.
 *
 * @param vertex - The vertex to expand from.
 * @param direction - Expansion direction. All directions (IN, OUT, BOTH)
 *    are supported.
 * @return See above.
 */
auto ExpandFromVertex(const VertexAccessor &vertex,
                      EdgeAtom::Direction direction,
                      const std::vector<storage::EdgeType> &edge_types) {
  // wraps an EdgeAccessor into a pair <accessor, direction>
  auto wrapper = [](EdgeAtom::Direction direction, auto &&vertices) {
    return iter::imap(
        [direction](const EdgeAccessor &edge) {
          return std::make_pair(edge, direction);
        },
        std::move(vertices));
  };

  // prepare a vector of elements we'll pass to the itertools
  std::vector<decltype(wrapper(direction, vertex.in()))> chain_elements;

  if (direction != EdgeAtom::Direction::OUT && vertex.in_degree() > 0) {
    auto edges = vertex.in(&edge_types);
    if (edges.begin() != edges.end()) {
      chain_elements.emplace_back(
          wrapper(EdgeAtom::Direction::IN, std::move(edges)));
    }
  }
  if (direction != EdgeAtom::Direction::IN && vertex.out_degree() > 0) {
    auto edges = vertex.out(&edge_types);
    if (edges.begin() != edges.end()) {
      chain_elements.emplace_back(
          wrapper(EdgeAtom::Direction::OUT, std::move(edges)));
    }
  }

  return iter::chain.from_iterable(std::move(chain_elements));
}

/** A helper function for evaluating an expression that's an int.
 *
 * @param evaluator
 * @param expr
 * @param what - Name of what's getting evaluated. Used for user
 * feedback (via exception) when the evaluated value is not an int.
 */
int64_t EvaluateInt(ExpressionEvaluator &evaluator, Expression *expr,
                    const std::string &what) {
  TypedValue value = expr->Accept(evaluator);
  try {
    return value.Value<int64_t>();
  } catch (TypedValueException &e) {
    throw QueryRuntimeException(what + " must be an int");
  }
}
}  // namespace

class ExpandVariableCursor : public Cursor {
 public:
  ExpandVariableCursor(const ExpandVariable &self,
                       database::GraphDbAccessor &db)
      : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

  bool Pull(Frame &frame, Context &context) override {
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_,
                                  self_.graph_view_);
    while (true) {
      if (Expand(frame, context)) return true;

      if (PullInput(frame, context)) {
        // if lower bound is zero we also yield empty paths
        if (lower_bound_ == 0) {
          auto &start_vertex =
              frame[self_.input_symbol_].Value<VertexAccessor>();
          if (self_.HandleExistingNode(start_vertex, frame)) {
            return true;
          }
        }
        // if lower bound is not zero, we just continue, the next
        // loop iteration will attempt to expand and we're good
      } else
        return false;
      // else continue with the loop, try to expand again
      // because we succesfully pulled from the input
    }
  }

  void Reset() override {
    input_cursor_->Reset();
    edges_.clear();
    edges_it_.clear();
  }

 private:
  const ExpandVariable &self_;
  database::GraphDbAccessor &db_;
  const std::unique_ptr<Cursor> input_cursor_;
  // bounds. in the cursor they are not optional but set to
  // default values if missing in the ExpandVariable operator
  // initialize to arbitrary values, they should only be used
  // after a successful pull from the input
  int64_t upper_bound_{-1};
  int64_t lower_bound_{-1};

  // a stack of edge iterables corresponding to the level/depth of
  // the expansion currently being Pulled
  std::vector<decltype(ExpandFromVertex(std::declval<VertexAccessor>(),
                                        EdgeAtom::Direction::IN,
                                        self_.edge_types_))>
      edges_;

  // an iterator indicating the possition in the corresponding edges_
  // element
  std::vector<decltype(edges_.begin()->begin())> edges_it_;

  /**
   * Helper function that Pulls from the input vertex and
   * makes iteration over it's edges possible.
   *
   * @return If the Pull succeeded. If not, this VariableExpandCursor
   * is exhausted.
   */
  bool PullInput(Frame &frame, Context &context) {
    // Input Vertex could be null if it is created by a failed optional
    // match.
    // In those cases we skip that input pull and continue with the next.
    while (true) {
      if (!input_cursor_->Pull(frame, context)) return false;
      TypedValue &vertex_value = frame[self_.input_symbol_];

      // Null check due to possible failed optional match.
      if (vertex_value.IsNull()) continue;

      ExpectType(self_.input_symbol_, vertex_value, TypedValue::Type::Vertex);
      auto &vertex = vertex_value.Value<VertexAccessor>();
      SwitchAccessor(vertex, self_.graph_view_);

      // Evaluate the upper and lower bounds.
      ExpressionEvaluator evaluator(frame, context.parameters_,
                                    context.symbol_table_, db_);
      auto calc_bound = [&evaluator](auto &bound) {
        auto value = EvaluateInt(evaluator, bound, "Variable expansion bound");
        if (value < 0)
          throw QueryRuntimeException(
              "Variable expansion bound must be positive or zero");
        return value;
      };

      lower_bound_ = self_.lower_bound_ ? calc_bound(self_.lower_bound_) : 1;
      upper_bound_ = self_.upper_bound_ ? calc_bound(self_.upper_bound_)
                                        : std::numeric_limits<int64_t>::max();

      if (upper_bound_ > 0) {
        SwitchAccessor(vertex, self_.graph_view_);
        edges_.emplace_back(
            ExpandFromVertex(vertex, self_.direction_, self_.edge_types_));
        edges_it_.emplace_back(edges_.back().begin());
      }

      // reset the frame value to an empty edge list
      frame[self_.edge_symbol_] = std::vector<TypedValue>();

      return true;
    }
  }

  // Helper function for appending an edge to the list on the frame.
  void AppendEdge(const EdgeAccessor &new_edge,
                  std::vector<TypedValue> &edges_on_frame) {
    // We are placing an edge on the frame. It is possible that there already
    // exists an edge on the frame for this level. If so first remove it.
    DCHECK(edges_.size() > 0) << "Edges are empty";
    if (self_.is_reverse_) {
      // TODO: This is innefficient, we should look into replacing
      // vector with something else for TypedValue::List.
      size_t diff = edges_on_frame.size() -
                    std::min(edges_on_frame.size(), edges_.size() - 1U);
      if (diff > 0U)
        edges_on_frame.erase(edges_on_frame.begin(),
                             edges_on_frame.begin() + diff);
      edges_on_frame.insert(edges_on_frame.begin(), new_edge);
    } else {
      edges_on_frame.resize(
          std::min(edges_on_frame.size(), edges_.size() - 1U));
      edges_on_frame.emplace_back(new_edge);
    }
  }

  /**
   * Performs a single expansion for the current state of this
   * VariableExpansionCursor.
   *
   * @return True if the expansion was a success and this Cursor's
   * consumer can consume it. False if the expansion failed. In that
   * case no more expansions are available from the current input
   * vertex and another Pull from the input cursor should be performed.
   */
  bool Expand(Frame &frame, Context &context) {
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_,
                                  self_.graph_view_);
    // Some expansions might not be valid due to edge uniqueness and
    // existing_node criterions, so expand in a loop until either the input
    // vertex is exhausted or a valid variable-length expansion is available.
    while (true) {
      // pop from the stack while there is stuff to pop and the current
      // level is exhausted
      while (!edges_.empty() && edges_it_.back() == edges_.back().end()) {
        edges_.pop_back();
        edges_it_.pop_back();
      }

      // check if we exhausted everything, if so return false
      if (edges_.empty()) return false;

      // we use this a lot
      std::vector<TypedValue> &edges_on_frame =
          frame[self_.edge_symbol_].Value<std::vector<TypedValue>>();

      // it is possible that edges_on_frame does not contain as many
      // elements as edges_ due to edge-uniqueness (when a whole layer
      // gets exhausted but no edges are valid). for that reason only
      // pop from edges_on_frame if they contain enough elements
      if (self_.is_reverse_) {
        auto diff = edges_on_frame.size() -
                    std::min(edges_on_frame.size(), edges_.size());
        if (diff > 0) {
          edges_on_frame.erase(edges_on_frame.begin(),
                               edges_on_frame.begin() + diff);
        }
      } else {
        edges_on_frame.resize(std::min(edges_on_frame.size(), edges_.size()));
      }

      // if we are here, we have a valid stack,
      // get the edge, increase the relevant iterator
      std::pair<EdgeAccessor, EdgeAtom::Direction> current_edge =
          *edges_it_.back()++;

      // Check edge-uniqueness.
      bool found_existing =
          std::any_of(edges_on_frame.begin(), edges_on_frame.end(),
                      [&current_edge](const TypedValue &edge) {
                        return current_edge.first == edge.Value<EdgeAccessor>();
                      });
      if (found_existing) continue;

      AppendEdge(current_edge.first, edges_on_frame);
      VertexAccessor current_vertex =
          current_edge.second == EdgeAtom::Direction::IN
              ? current_edge.first.from()
              : current_edge.first.to();

      if (!self_.HandleExistingNode(current_vertex, frame)) continue;

      // Skip expanding out of filtered expansion.
      frame[self_.filter_lambda_.inner_edge_symbol] = current_edge.first;
      frame[self_.filter_lambda_.inner_node_symbol] = current_vertex;
      if (self_.filter_lambda_.expression &&
          !EvaluateFilter(evaluator, self_.filter_lambda_.expression))
        continue;

      // we are doing depth-first search, so place the current
      // edge's expansions onto the stack, if we should continue to expand
      if (upper_bound_ > static_cast<int64_t>(edges_.size())) {
        SwitchAccessor(current_vertex, self_.graph_view_);
        edges_.emplace_back(ExpandFromVertex(current_vertex, self_.direction_,
                                             self_.edge_types_));
        edges_it_.emplace_back(edges_.back().begin());
      }

      // We only yield true if we satisfy the lower bound.
      if (static_cast<int64_t>(edges_on_frame.size()) >= lower_bound_)
        return true;
      else
        continue;
    }
  }
};

class ExpandBreadthFirstCursor : public query::plan::Cursor {
 public:
  ExpandBreadthFirstCursor(const ExpandVariable &self,
                           database::GraphDbAccessor &db)
      : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

  bool Pull(Frame &frame, Context &context) override {
    // evaulator for the filtering condition
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_,
                                  self_.graph_view_);

    // for the given (edge, vertex) pair checks if they satisfy the
    // "where" condition. if so, places them in the to_visit_ structure.
    auto expand_pair = [this, &evaluator, &frame](EdgeAccessor edge,
                                                  VertexAccessor vertex) {
      // if we already processed the given vertex it doesn't get expanded
      if (processed_.find(vertex) != processed_.end()) return;

      SwitchAccessor(edge, self_.graph_view_);
      SwitchAccessor(vertex, self_.graph_view_);

      frame[self_.filter_lambda_.inner_edge_symbol] = edge;
      frame[self_.filter_lambda_.inner_node_symbol] = vertex;

      if (self_.filter_lambda_.expression) {
        TypedValue result = self_.filter_lambda_.expression->Accept(evaluator);
        switch (result.type()) {
          case TypedValue::Type::Null:
            return;
          case TypedValue::Type::Bool:
            if (!result.Value<bool>()) return;
            break;
          default:
            throw QueryRuntimeException(
                "Expansion condition must be boolean or null");
        }
      }
      to_visit_next_.emplace_back(edge, vertex);
      processed_.emplace(vertex, edge);
    };

    // populates the to_visit_next_ structure with expansions
    // from the given vertex. skips expansions that don't satisfy
    // the "where" condition.
    auto expand_from_vertex = [this, &expand_pair](VertexAccessor &vertex) {
      if (self_.direction_ != EdgeAtom::Direction::IN) {
        for (const EdgeAccessor &edge : vertex.out(&self_.edge_types_))
          expand_pair(edge, edge.to());
      }
      if (self_.direction_ != EdgeAtom::Direction::OUT) {
        for (const EdgeAccessor &edge : vertex.in(&self_.edge_types_))
          expand_pair(edge, edge.from());
      }
    };

    // do it all in a loop because we skip some elements
    while (true) {
      // if we have nothing to visit on the current depth, switch to next
      if (to_visit_current_.empty()) to_visit_current_.swap(to_visit_next_);

      // if current is still empty, it means both are empty, so pull from
      // input
      if (to_visit_current_.empty()) {
        if (!input_cursor_->Pull(frame, context)) return false;
        processed_.clear();

        auto vertex_value = frame[self_.input_symbol_];
        // it is possible that the vertex is Null due to optional matching
        if (vertex_value.IsNull()) continue;
        auto vertex = vertex_value.Value<VertexAccessor>();
        SwitchAccessor(vertex, self_.graph_view_);
        processed_.emplace(vertex, std::experimental::nullopt);
        expand_from_vertex(vertex);
        lower_bound_ = self_.lower_bound_
                           ? EvaluateInt(evaluator, self_.lower_bound_,
                                         "Min depth in breadth-first expansion")
                           : 1;
        upper_bound_ = self_.upper_bound_
                           ? EvaluateInt(evaluator, self_.upper_bound_,
                                         "Max depth in breadth-first expansion")
                           : std::numeric_limits<int>::max();
        if (upper_bound_ < 1)
          throw QueryRuntimeException(
              "Max depth in breadth-first expansion must be greater then "
              "zero");

        // go back to loop start and see if we expanded anything
        continue;
      }

      // take the next expansion from the queue
      std::pair<EdgeAccessor, VertexAccessor> expansion =
          to_visit_current_.front();
      to_visit_current_.pop_front();

      // create the frame value for the edges
      std::vector<TypedValue> edge_list{expansion.first};
      auto last_vertex = expansion.second;
      while (true) {
        const EdgeAccessor &last_edge = edge_list.back().Value<EdgeAccessor>();
        last_vertex =
            last_edge.from() == last_vertex ? last_edge.to() : last_edge.from();
        // origin_vertex must be in processed
        const auto &previous_edge = processed_.find(last_vertex)->second;
        if (!previous_edge) break;

        edge_list.push_back(previous_edge.value());
      }

      // expand only if what we've just expanded is less then max depth
      if (static_cast<int>(edge_list.size()) < upper_bound_)
        expand_from_vertex(expansion.second);

      if (static_cast<int64_t>(edge_list.size()) < lower_bound_) continue;

      // place destination node on the frame, handle existence flag
      if (self_.existing_node_) {
        TypedValue &node = frame[self_.node_symbol_];
        // due to optional matching the existing node could be null
        if (node.IsNull() || (node != expansion.second).Value<bool>()) continue;
      } else
        frame[self_.node_symbol_] = expansion.second;

      // place edges on the frame in the correct order
      std::reverse(edge_list.begin(), edge_list.end());
      frame[self_.edge_symbol_] = std::move(edge_list);

      return true;
    }
  }
  void Reset() override {
    input_cursor_->Reset();
    processed_.clear();
    to_visit_next_.clear();
    to_visit_current_.clear();
  }

 private:
  const ExpandVariable &self_;
  database::GraphDbAccessor &db_;
  const std::unique_ptr<query::plan::Cursor> input_cursor_;

  // Depth bounds. Calculated on each pull from the input, the initial value is
  // irrelevant.
  int lower_bound_{-1};
  int upper_bound_{-1};

  // maps vertices to the edge they got expanded from. it is an optional
  // edge because the root does not get expanded from anything.
  // contains visited vertices as well as those scheduled to be visited.
  std::unordered_map<VertexAccessor, std::experimental::optional<EdgeAccessor>>
      processed_;
  // edge/vertex pairs we have yet to visit, for current and next depth
  std::deque<std::pair<EdgeAccessor, VertexAccessor>> to_visit_current_;
  std::deque<std::pair<EdgeAccessor, VertexAccessor>> to_visit_next_;
};

class ExpandWeightedShortestPathCursor : public query::plan::Cursor {
 public:
  ExpandWeightedShortestPathCursor(const ExpandVariable &self,
                                   database::GraphDbAccessor &db)
      : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

  bool Pull(Frame &frame, Context &context) override {
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_,
                                  self_.graph_view_);
    // For the given (vertex, edge, vertex) tuple checks if they satisfy the
    // "where" condition. if so, places them in the priority queue.
    auto expand_pair = [this, &evaluator, &frame](
        VertexAccessor from, EdgeAccessor edge, VertexAccessor vertex) {
      SwitchAccessor(edge, self_.graph_view_);
      SwitchAccessor(vertex, self_.graph_view_);

      if (self_.filter_lambda_.expression) {
        frame[self_.filter_lambda_.inner_edge_symbol] = edge;
        frame[self_.filter_lambda_.inner_node_symbol] = vertex;

        if (!EvaluateFilter(evaluator, self_.filter_lambda_.expression)) return;
      }

      frame[self_.weight_lambda_->inner_edge_symbol] = edge;
      frame[self_.weight_lambda_->inner_node_symbol] = vertex;

      TypedValue typed_weight =
          self_.weight_lambda_->expression->Accept(evaluator);

      if (!typed_weight.IsNumeric()) {
        throw QueryRuntimeException("Calculated weight must be numeric, got {}",
                                    typed_weight.type());
      }
      if ((typed_weight < 0).Value<bool>()) {
        throw QueryRuntimeException("Calculated weight can't be negative!");
      }

      auto total_weight = weights_[from] + typed_weight;
      auto found_it = weights_.find(vertex);
      if (found_it != weights_.end() &&
          found_it->second.Value<double>() <= total_weight.Value<double>())
        return;

      pq_.push(std::make_pair(std::make_pair(vertex, edge),
                              total_weight.Value<double>()));
    };

    // Populates the priority queue structure with expansions
    // from the given vertex. skips expansions that don't satisfy
    // the "where" condition.
    auto expand_from_vertex = [this, &expand_pair](VertexAccessor &vertex) {
      if (self_.direction_ != EdgeAtom::Direction::IN) {
        for (const EdgeAccessor &edge : vertex.out(&self_.edge_types_)) {
          expand_pair(vertex, edge, edge.to());
        }
      }
      if (self_.direction_ != EdgeAtom::Direction::OUT) {
        for (const EdgeAccessor &edge : vertex.in(&self_.edge_types_)) {
          expand_pair(vertex, edge, edge.from());
        }
      }
    };

    while (true) {
      if (pq_.empty()) {
        if (!input_cursor_->Pull(frame, context)) return false;
        auto vertex_value = frame[self_.input_symbol_];
        if (vertex_value.IsNull()) continue;
        auto vertex = vertex_value.Value<VertexAccessor>();
        if (self_.existing_node_) {
          TypedValue &node = frame[self_.node_symbol_];
          // Due to optional matching the existing node could be null.
          // Skip expansion for such nodes.
          if (node.IsNull()) continue;
        }
        SwitchAccessor(vertex, self_.graph_view_);
        upper_bound_ =
            self_.upper_bound_
                ? EvaluateInt(evaluator, self_.upper_bound_,
                              "Max depth in weighted shortest path expansion")
                : std::numeric_limits<int>::max();
        if (upper_bound_ < 1)
          throw QueryRuntimeException(
              "Max depth in weighted shortest path expansion must be greater "
              "than zero");

        // Clear existing data structures.
        previous_.clear();
        weights_.clear();

        pq_.push(std::make_pair(
            std::make_pair(vertex, std::experimental::nullopt), 0.0));
      }

      while (!pq_.empty()) {
        auto current = pq_.top();
        pq_.pop();

        // Check if the edge has already been processed.
        if (weights_.find(current.first.first) != weights_.end()) {
          continue;
        }
        previous_.emplace(current.first.first, current.first.second);
        weights_.emplace(current.first.first, current.second);

        // Reconstruct the path.
        auto last_vertex = current.first.first;
        std::vector<TypedValue> edge_list{};
        while (true) {
          // Origin_vertex must be in previous.
          const auto &previous_edge = previous_.find(last_vertex)->second;
          if (!previous_edge) break;
          last_vertex = previous_edge->from() == last_vertex
                            ? previous_edge->to()
                            : previous_edge->from();
          edge_list.push_back(previous_edge.value());
        }

        // Expand only if what we've just expanded is less then max depth.
        if (static_cast<int>(edge_list.size()) < upper_bound_)
          expand_from_vertex(current.first.first);

        if (edge_list.empty()) continue;

        // Place destination node on the frame, handle existence flag.
        if (self_.existing_node_) {
          TypedValue &node = frame[self_.node_symbol_];
          if ((node != current.first.first).Value<bool>())
            continue;
          else
            // Prevent expanding other paths, because we found the
            // shortest to existing node.
            ClearQueue();
        } else {
          frame[self_.node_symbol_] = current.first.first;
        }

        if (!self_.is_reverse_) {
          // Place edges on the frame in the correct order.
          std::reverse(edge_list.begin(), edge_list.end());
        }
        frame[self_.edge_symbol_] = std::move(edge_list);
        frame[self_.total_weight_.value()] = current.second;
        return true;
      }
    }
  }

  void Reset() override {
    input_cursor_->Reset();
    previous_.clear();
    weights_.clear();
    ClearQueue();
  }

 private:
  const ExpandVariable &self_;
  database::GraphDbAccessor &db_;
  const std::unique_ptr<query::plan::Cursor> input_cursor_;

  // Upper bound on the path length.
  int upper_bound_{-1};

  // Maps vertices to weights they got in expansion.
  std::unordered_map<VertexAccessor, TypedValue> weights_;

  // Maps vertices to edges used to reach them.
  std::unordered_map<VertexAccessor, std::experimental::optional<EdgeAccessor>>
      previous_;

  // Priority queue comparator. Keep lowest weight on top of the queue.
  class PriorityQueueComparator {
   public:
    bool operator()(
        const std::pair<std::pair<VertexAccessor,
                                  std::experimental::optional<EdgeAccessor>>,
                        double> &lhs,
        const std::pair<std::pair<VertexAccessor,
                                  std::experimental::optional<EdgeAccessor>>,
                        double> &rhs) {
      return lhs.second > rhs.second;
    }
  };
  std::priority_queue<
      std::pair<
          std::pair<VertexAccessor, std::experimental::optional<EdgeAccessor>>,
          double>,
      std::vector<std::pair<
          std::pair<VertexAccessor, std::experimental::optional<EdgeAccessor>>,
          double>>,
      PriorityQueueComparator>
      pq_;

  void ClearQueue() {
    while (!pq_.empty()) pq_.pop();
  }
};

std::unique_ptr<Cursor> ExpandVariable::MakeCursor(
    database::GraphDbAccessor &db) const {
  if (type_ == EdgeAtom::Type::BREADTH_FIRST)
    return std::make_unique<ExpandBreadthFirstCursor>(*this, db);
  else if (type_ == EdgeAtom::Type::WEIGHTED_SHORTEST_PATH)
    return std::make_unique<ExpandWeightedShortestPathCursor>(*this, db);
  else
    return std::make_unique<ExpandVariableCursor>(*this, db);
}

class ConstructNamedPathCursor : public Cursor {
 public:
  ConstructNamedPathCursor(const ConstructNamedPath &self,
                           database::GraphDbAccessor &db)
      : self_(self), input_cursor_(self_.input()->MakeCursor(db)) {}

  bool Pull(Frame &frame, Context &context) override {
    if (!input_cursor_->Pull(frame, context)) return false;

    auto symbol_it = self_.path_elements().begin();
    DCHECK(symbol_it != self_.path_elements().end())
        << "Named path must contain at least one node";

    TypedValue start_vertex = frame[*symbol_it++];

    // In an OPTIONAL MATCH everything could be Null.
    if (start_vertex.IsNull()) {
      frame[self_.path_symbol()] = TypedValue::Null;
      return true;
    }

    DCHECK(start_vertex.IsVertex())
        << "First named path element must be a vertex";
    query::Path path(start_vertex.ValueVertex());

    // If the last path element symbol was for an edge list, then
    // the next symbol is a vertex and it should not append to the path
    // because
    // expansion already did it.
    bool last_was_edge_list = false;

    for (; symbol_it != self_.path_elements().end(); symbol_it++) {
      TypedValue expansion = frame[*symbol_it];
      //  We can have Null (OPTIONAL MATCH), a vertex, an edge, or an edge
      //  list (variable expand or BFS).
      switch (expansion.type()) {
        case TypedValue::Type::Null:
          frame[self_.path_symbol()] = TypedValue::Null;
          return true;
        case TypedValue::Type::Vertex:
          if (!last_was_edge_list) path.Expand(expansion.ValueVertex());
          last_was_edge_list = false;
          break;
        case TypedValue::Type::Edge:
          path.Expand(expansion.ValueEdge());
          break;
        case TypedValue::Type::List: {
          last_was_edge_list = true;
          // We need to expand all edges in the list and intermediary
          // vertices.
          const std::vector<TypedValue> &edges = expansion.ValueList();
          for (const auto &edge_value : edges) {
            const EdgeAccessor &edge = edge_value.ValueEdge();
            const VertexAccessor from = edge.from();
            if (path.vertices().back() == from)
              path.Expand(edge, edge.to());
            else
              path.Expand(edge, from);
          }
          break;
        }
        default:
          LOG(FATAL) << "Unsupported type in named path construction";

          break;
      }
    }

    frame[self_.path_symbol()] = path;
    return true;
  }

  void Reset() override { input_cursor_->Reset(); }

 private:
  const ConstructNamedPath self_;
  const std::unique_ptr<Cursor> input_cursor_;
};

ACCEPT_WITH_INPUT(ConstructNamedPath)

std::unique_ptr<Cursor> ConstructNamedPath::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<ConstructNamedPathCursor>(*this, db);
}

std::vector<Symbol> ConstructNamedPath::ModifiedSymbols(
    const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(path_symbol_);
  return symbols;
}

Filter::Filter(const std::shared_ptr<LogicalOperator> &input,
               Expression *expression)
    : input_(input ? input : std::make_shared<Once>()),
      expression_(expression) {}

ACCEPT_WITH_INPUT(Filter)

std::unique_ptr<Cursor> Filter::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<FilterCursor>(*this, db);
}

std::vector<Symbol> Filter::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

Filter::FilterCursor::FilterCursor(const Filter &self,
                                   database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

bool Filter::FilterCursor::Pull(Frame &frame, Context &context) {
  // Like all filters, newly set values should not affect filtering of old
  // nodes and edges.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::OLD);
  while (input_cursor_->Pull(frame, context)) {
    if (EvaluateFilter(evaluator, self_.expression_)) return true;
  }
  return false;
}

void Filter::FilterCursor::Reset() { input_cursor_->Reset(); }

Produce::Produce(const std::shared_ptr<LogicalOperator> &input,
                 const std::vector<NamedExpression *> &named_expressions)
    : input_(input ? input : std::make_shared<Once>()),
      named_expressions_(named_expressions) {}

ACCEPT_WITH_INPUT(Produce)

std::unique_ptr<Cursor> Produce::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<ProduceCursor>(*this, db);
}

std::vector<Symbol> Produce::OutputSymbols(
    const SymbolTable &symbol_table) const {
  std::vector<Symbol> symbols;
  for (const auto &named_expr : named_expressions_) {
    symbols.emplace_back(symbol_table.at(*named_expr));
  }
  return symbols;
}

std::vector<Symbol> Produce::ModifiedSymbols(const SymbolTable &table) const {
  return OutputSymbols(table);
}

const std::vector<NamedExpression *> &Produce::named_expressions() {
  return named_expressions_;
}

Produce::ProduceCursor::ProduceCursor(const Produce &self,
                                      database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

bool Produce::ProduceCursor::Pull(Frame &frame, Context &context) {
  if (input_cursor_->Pull(frame, context)) {
    // Produce should always yield the latest results.
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_, GraphView::NEW);
    for (auto named_expr : self_.named_expressions_)
      named_expr->Accept(evaluator);
    return true;
  }
  return false;
}

void Produce::ProduceCursor::Reset() { input_cursor_->Reset(); }

Delete::Delete(const std::shared_ptr<LogicalOperator> &input_,
               const std::vector<Expression *> &expressions, bool detach_)
    : input_(input_), expressions_(expressions), detach_(detach_) {}

ACCEPT_WITH_INPUT(Delete)

std::unique_ptr<Cursor> Delete::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<DeleteCursor>(*this, db);
}

std::vector<Symbol> Delete::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

Delete::DeleteCursor::DeleteCursor(const Delete &self,
                                   database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

bool Delete::DeleteCursor::Pull(Frame &frame, Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  // Delete should get the latest information, this way it is also possible
  // to
  // delete newly added nodes and edges.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::NEW);
  // collect expressions results so edges can get deleted before vertices
  // this is necessary because an edge that gets deleted could block vertex
  // deletion
  std::vector<TypedValue> expression_results;
  expression_results.reserve(self_.expressions_.size());
  for (Expression *expression : self_.expressions_) {
    expression_results.emplace_back(expression->Accept(evaluator));
  }

  // delete edges first
  for (TypedValue &expression_result : expression_results)
    if (expression_result.type() == TypedValue::Type::Edge)
      db_.RemoveEdge(expression_result.Value<EdgeAccessor>());

  // delete vertices
  for (TypedValue &expression_result : expression_results)
    switch (expression_result.type()) {
      case TypedValue::Type::Vertex: {
        VertexAccessor &va = expression_result.Value<VertexAccessor>();
        va.SwitchNew();  //  necessary because an edge deletion could have
                         //  updated
        if (self_.detach_)
          db_.DetachRemoveVertex(va);
        else if (!db_.RemoveVertex(va))
          throw RemoveAttachedVertexException();
        break;
      }

      // skip Edges (already deleted) and Nulls (can occur in optional
      // match)
      case TypedValue::Type::Edge:
      case TypedValue::Type::Null:
        break;
      // check we're not trying to delete anything except vertices and edges
      default:
        throw QueryRuntimeException("Can only delete edges and vertices");
    }

  return true;
}

void Delete::DeleteCursor::Reset() { input_cursor_->Reset(); }

SetProperty::SetProperty(const std::shared_ptr<LogicalOperator> &input,
                         PropertyLookup *lhs, Expression *rhs)
    : input_(input), lhs_(lhs), rhs_(rhs) {}

ACCEPT_WITH_INPUT(SetProperty)

std::unique_ptr<Cursor> SetProperty::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<SetPropertyCursor>(*this, db);
}

std::vector<Symbol> SetProperty::ModifiedSymbols(
    const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

SetProperty::SetPropertyCursor::SetPropertyCursor(const SetProperty &self,
                                                  database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool SetProperty::SetPropertyCursor::Pull(Frame &frame, Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  // Set, just like Create needs to see the latest changes.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::NEW);
  TypedValue lhs = self_.lhs_->expression_->Accept(evaluator);
  TypedValue rhs = self_.rhs_->Accept(evaluator);

  switch (lhs.type()) {
    case TypedValue::Type::Vertex:
      PropsSetChecked(lhs.Value<VertexAccessor>(), self_.lhs_->property_, rhs);
      break;
    case TypedValue::Type::Edge:
      PropsSetChecked(lhs.Value<EdgeAccessor>(), self_.lhs_->property_, rhs);
      break;
    case TypedValue::Type::Null:
      // Skip setting properties on Null (can occur in optional match).
      break;
    case TypedValue::Type::Map:
    // Semantically modifying a map makes sense, but it's not supported due
    // to
    // all the copying we do (when PropertyValue -> TypedValue and in
    // ExpressionEvaluator). So even though we set a map property here, that
    // is never visible to the user and it's not stored.
    // TODO: fix above described bug
    default:
      throw QueryRuntimeException(
          "Properties can only be set on Vertices and Edges");
  }
  return true;
}

void SetProperty::SetPropertyCursor::Reset() { input_cursor_->Reset(); }

SetProperties::SetProperties(const std::shared_ptr<LogicalOperator> &input,
                             Symbol input_symbol, Expression *rhs, Op op)
    : input_(input), input_symbol_(input_symbol), rhs_(rhs), op_(op) {}

ACCEPT_WITH_INPUT(SetProperties)

std::unique_ptr<Cursor> SetProperties::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<SetPropertiesCursor>(*this, db);
}

std::vector<Symbol> SetProperties::ModifiedSymbols(
    const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

SetProperties::SetPropertiesCursor::SetPropertiesCursor(
    const SetProperties &self, database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool SetProperties::SetPropertiesCursor::Pull(Frame &frame, Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  TypedValue &lhs = frame[self_.input_symbol_];

  // Set, just like Create needs to see the latest changes.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::NEW);
  TypedValue rhs = self_.rhs_->Accept(evaluator);

  switch (lhs.type()) {
    case TypedValue::Type::Vertex:
      Set(lhs.Value<VertexAccessor>(), rhs);
      break;
    case TypedValue::Type::Edge:
      Set(lhs.Value<EdgeAccessor>(), rhs);
      break;
    case TypedValue::Type::Null:
      // Skip setting properties on Null (can occur in optional match).
      break;
    default:
      throw QueryRuntimeException(
          "Properties can only be set on Vertices and Edges");
  }
  return true;
}

void SetProperties::SetPropertiesCursor::Reset() { input_cursor_->Reset(); }

template <typename TRecordAccessor>
void SetProperties::SetPropertiesCursor::Set(TRecordAccessor &record,
                                             const TypedValue &rhs) const {
  record.SwitchNew();
  if (self_.op_ == Op::REPLACE) {
    try {
      record.PropsClear();
    } catch (const RecordDeletedError &) {
      throw QueryRuntimeException(
          "Trying to set properties on a deleted graph element.");
    }
  }

  auto set_props = [&record](const auto &properties) {
    try {
      for (const auto &kv : properties) record.PropsSet(kv.first, kv.second);
    } catch (const RecordDeletedError &) {
      throw QueryRuntimeException(
          "Trying to set properties on a deleted graph element.");
    }
  };

  switch (rhs.type()) {
    case TypedValue::Type::Edge:
      set_props(rhs.Value<EdgeAccessor>().Properties());
      break;
    case TypedValue::Type::Vertex:
      set_props(rhs.Value<VertexAccessor>().Properties());
      break;
    case TypedValue::Type::Map: {
      for (const auto &kv : rhs.Value<std::map<std::string, TypedValue>>())
        PropsSetChecked(record, db_.Property(kv.first), kv.second);
      break;
    }
    default:
      throw QueryRuntimeException(
          "Can only set Vertices, Edges and maps as properties");
  }
}

// instantiate the SetProperties function with concrete TRecordAccessor
// types
template void SetProperties::SetPropertiesCursor::Set(
    RecordAccessor<Vertex> &record, const TypedValue &rhs) const;
template void SetProperties::SetPropertiesCursor::Set(
    RecordAccessor<Edge> &record, const TypedValue &rhs) const;

SetLabels::SetLabels(const std::shared_ptr<LogicalOperator> &input,
                     Symbol input_symbol,
                     const std::vector<storage::Label> &labels)
    : input_(input), input_symbol_(input_symbol), labels_(labels) {}

ACCEPT_WITH_INPUT(SetLabels)

std::unique_ptr<Cursor> SetLabels::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<SetLabelsCursor>(*this, db);
}

std::vector<Symbol> SetLabels::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

SetLabels::SetLabelsCursor::SetLabelsCursor(const SetLabels &self,
                                            database::GraphDbAccessor &db)
    : self_(self), input_cursor_(self.input_->MakeCursor(db)) {}

bool SetLabels::SetLabelsCursor::Pull(Frame &frame, Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  TypedValue &vertex_value = frame[self_.input_symbol_];
  // Skip setting labels on Null (can occur in optional match).
  if (vertex_value.IsNull()) return true;
  ExpectType(self_.input_symbol_, vertex_value, TypedValue::Type::Vertex);
  auto &vertex = vertex_value.Value<VertexAccessor>();
  vertex.SwitchNew();
  try {
    for (auto label : self_.labels_) vertex.add_label(label);
  } catch (const RecordDeletedError &) {
    throw QueryRuntimeException("Trying to set labels on a deleted Vertex");
  }

  return true;
}

void SetLabels::SetLabelsCursor::Reset() { input_cursor_->Reset(); }

RemoveProperty::RemoveProperty(const std::shared_ptr<LogicalOperator> &input,
                               PropertyLookup *lhs)
    : input_(input), lhs_(lhs) {}

ACCEPT_WITH_INPUT(RemoveProperty)

std::unique_ptr<Cursor> RemoveProperty::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<RemovePropertyCursor>(*this, db);
}

std::vector<Symbol> RemoveProperty::ModifiedSymbols(
    const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

RemoveProperty::RemovePropertyCursor::RemovePropertyCursor(
    const RemoveProperty &self, database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool RemoveProperty::RemovePropertyCursor::Pull(Frame &frame,
                                                Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  // Remove, just like Delete needs to see the latest changes.
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::NEW);
  TypedValue lhs = self_.lhs_->expression_->Accept(evaluator);

  switch (lhs.type()) {
    case TypedValue::Type::Vertex:
      try {
        lhs.Value<VertexAccessor>().PropsErase(self_.lhs_->property_);
      } catch (const RecordDeletedError &) {
        throw QueryRuntimeException(
            "Trying to remove properties from a deleted Vertex");
      }
      break;
    case TypedValue::Type::Edge:
      try {
        lhs.Value<EdgeAccessor>().PropsErase(self_.lhs_->property_);
      } catch (const RecordDeletedError &) {
        throw QueryRuntimeException(
            "Trying to remove properties from a deleted Edge");
      }
      break;
    case TypedValue::Type::Null:
      // Skip removing properties on Null (can occur in optional match).
      break;
    default:
      throw QueryRuntimeException(
          "Properties can only be removed on Vertices and Edges");
  }
  return true;
}

void RemoveProperty::RemovePropertyCursor::Reset() { input_cursor_->Reset(); }

RemoveLabels::RemoveLabels(const std::shared_ptr<LogicalOperator> &input,
                           Symbol input_symbol,
                           const std::vector<storage::Label> &labels)
    : input_(input), input_symbol_(input_symbol), labels_(labels) {}

ACCEPT_WITH_INPUT(RemoveLabels)

std::unique_ptr<Cursor> RemoveLabels::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<RemoveLabelsCursor>(*this, db);
}

std::vector<Symbol> RemoveLabels::ModifiedSymbols(
    const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

RemoveLabels::RemoveLabelsCursor::RemoveLabelsCursor(
    const RemoveLabels &self, database::GraphDbAccessor &db)
    : self_(self), input_cursor_(self.input_->MakeCursor(db)) {}

bool RemoveLabels::RemoveLabelsCursor::Pull(Frame &frame, Context &context) {
  if (!input_cursor_->Pull(frame, context)) return false;

  TypedValue &vertex_value = frame[self_.input_symbol_];
  // Skip removing labels on Null (can occur in optional match).
  if (vertex_value.IsNull()) return true;
  ExpectType(self_.input_symbol_, vertex_value, TypedValue::Type::Vertex);
  auto &vertex = vertex_value.Value<VertexAccessor>();
  vertex.SwitchNew();
  try {
    for (auto label : self_.labels_) vertex.remove_label(label);
  } catch (const RecordDeletedError &) {
    throw QueryRuntimeException(
        "Trying to remove labels from a deleted Vertex");
  }

  return true;
}

void RemoveLabels::RemoveLabelsCursor::Reset() { input_cursor_->Reset(); }

template <typename TAccessor>
ExpandUniquenessFilter<TAccessor>::ExpandUniquenessFilter(
    const std::shared_ptr<LogicalOperator> &input, Symbol expand_symbol,
    const std::vector<Symbol> &previous_symbols)
    : input_(input),
      expand_symbol_(expand_symbol),
      previous_symbols_(previous_symbols) {}

template <typename TAccessor>
ACCEPT_WITH_INPUT(ExpandUniquenessFilter<TAccessor>)

template <typename TAccessor>
std::unique_ptr<Cursor> ExpandUniquenessFilter<TAccessor>::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<ExpandUniquenessFilterCursor>(*this, db);
}

template <typename TAccessor>
std::vector<Symbol> ExpandUniquenessFilter<TAccessor>::ModifiedSymbols(
    const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

template <typename TAccessor>
ExpandUniquenessFilter<TAccessor>::ExpandUniquenessFilterCursor::
    ExpandUniquenessFilterCursor(const ExpandUniquenessFilter &self,
                                 database::GraphDbAccessor &db)
    : self_(self), input_cursor_(self.input_->MakeCursor(db)) {}

namespace {
/**
 * Returns true if:
 *    - a and b are vertex values and are the same
 *    - a and b are either edge or edge-list values, and there
 *    is at least one matching edge in the two values
 */
template <typename TAccessor>
bool ContainsSame(const TypedValue &a, const TypedValue &b);

template <>
bool ContainsSame<VertexAccessor>(const TypedValue &a, const TypedValue &b) {
  return a.Value<VertexAccessor>() == b.Value<VertexAccessor>();
}

template <>
bool ContainsSame<EdgeAccessor>(const TypedValue &a, const TypedValue &b) {
  auto compare_to_list = [](const TypedValue &list, const TypedValue &other) {
    for (const TypedValue &list_elem : list.Value<std::vector<TypedValue>>())
      if (ContainsSame<EdgeAccessor>(list_elem, other)) return true;
    return false;
  };

  if (a.type() == TypedValue::Type::List) return compare_to_list(a, b);
  if (b.type() == TypedValue::Type::List) return compare_to_list(b, a);

  return a.Value<EdgeAccessor>() == b.Value<EdgeAccessor>();
}
}  // namespace

template <typename TAccessor>
bool ExpandUniquenessFilter<TAccessor>::ExpandUniquenessFilterCursor::Pull(
    Frame &frame, Context &context) {
  auto expansion_ok = [&]() {
    TypedValue &expand_value = frame[self_.expand_symbol_];
    for (const auto &previous_symbol : self_.previous_symbols_) {
      TypedValue &previous_value = frame[previous_symbol];
      // This shouldn't raise a TypedValueException, because the planner
      // makes sure these are all of the expected type. In case they are not
      // an error should be raised long before this code is executed.
      if (ContainsSame<TAccessor>(previous_value, expand_value)) return false;
    }
    return true;
  };

  while (input_cursor_->Pull(frame, context))
    if (expansion_ok()) return true;
  return false;
}

template <typename TAccessor>
void ExpandUniquenessFilter<TAccessor>::ExpandUniquenessFilterCursor::Reset() {
  input_cursor_->Reset();
}

// instantiations of the ExpandUniquenessFilter template class
// we only ever need these two
template class ExpandUniquenessFilter<VertexAccessor>;
template class ExpandUniquenessFilter<EdgeAccessor>;

Accumulate::Accumulate(const std::shared_ptr<LogicalOperator> &input,
                       const std::vector<Symbol> &symbols, bool advance_command)
    : input_(input), symbols_(symbols), advance_command_(advance_command) {}

ACCEPT_WITH_INPUT(Accumulate)

std::unique_ptr<Cursor> Accumulate::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<Accumulate::AccumulateCursor>(*this, db);
}

std::vector<Symbol> Accumulate::ModifiedSymbols(const SymbolTable &) const {
  return symbols_;
}

Accumulate::AccumulateCursor::AccumulateCursor(const Accumulate &self,
                                               database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool Accumulate::AccumulateCursor::Pull(Frame &frame, Context &context) {
  // cache all the input
  if (!pulled_all_input_) {
    while (input_cursor_->Pull(frame, context)) {
      std::vector<TypedValue> row;
      row.reserve(self_.symbols_.size());
      for (const Symbol &symbol : self_.symbols_)
        row.emplace_back(frame[symbol]);
      cache_.emplace_back(std::move(row));
    }
    pulled_all_input_ = true;
    cache_it_ = cache_.begin();

    if (self_.advance_command_) {
      db_.AdvanceCommand();
      for (auto &row : cache_)
        for (auto &col : row) query::ReconstructTypedValue(col);
    }
  }

  if (cache_it_ == cache_.end()) return false;
  auto row_it = (cache_it_++)->begin();
  for (const Symbol &symbol : self_.symbols_) frame[symbol] = *row_it++;
  return true;
}

void Accumulate::AccumulateCursor::Reset() {
  input_cursor_->Reset();
  cache_.clear();
  cache_it_ = cache_.begin();
  pulled_all_input_ = false;
}

Aggregate::Aggregate(const std::shared_ptr<LogicalOperator> &input,
                     const std::vector<Aggregate::Element> &aggregations,
                     const std::vector<Expression *> &group_by,
                     const std::vector<Symbol> &remember)
    : input_(input ? input : std::make_shared<Once>()),
      aggregations_(aggregations),
      group_by_(group_by),
      remember_(remember) {}

ACCEPT_WITH_INPUT(Aggregate)

std::unique_ptr<Cursor> Aggregate::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<AggregateCursor>(*this, db);
}

std::vector<Symbol> Aggregate::ModifiedSymbols(const SymbolTable &) const {
  auto symbols = remember_;
  for (const auto &elem : aggregations_) symbols.push_back(elem.output_sym);
  return symbols;
}

Aggregate::AggregateCursor::AggregateCursor(const Aggregate &self,
                                            database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

namespace {
/** Returns the default TypedValue for an Aggregation element.
 * This value is valid both for returning when where are no inputs
 * to the aggregation op, and for initializing an aggregation result
 * when there are */
TypedValue DefaultAggregationOpValue(const Aggregate::Element &element) {
  switch (element.op) {
    case Aggregation::Op::COUNT:
      return TypedValue(0);
    case Aggregation::Op::SUM:
    case Aggregation::Op::MIN:
    case Aggregation::Op::MAX:
    case Aggregation::Op::AVG:
      return TypedValue::Null;
    case Aggregation::Op::COLLECT_LIST:
      return TypedValue(std::vector<TypedValue>());
    case Aggregation::Op::COLLECT_MAP:
      return TypedValue(std::map<std::string, TypedValue>());
  }
}
}  // namespace

bool Aggregate::AggregateCursor::Pull(Frame &frame, Context &context) {
  if (!pulled_all_input_) {
    ProcessAll(frame, context);
    pulled_all_input_ = true;
    aggregation_it_ = aggregation_.begin();

    // in case there is no input and no group_bys we need to return true
    // just this once
    if (aggregation_.empty() && self_.group_by_.empty()) {
      // place default aggregation values on the frame
      for (const auto &elem : self_.aggregations_)
        frame[elem.output_sym] = DefaultAggregationOpValue(elem);
      // place null as remember values on the frame
      for (const Symbol &remember_sym : self_.remember_)
        frame[remember_sym] = TypedValue::Null;
      return true;
    }
  }

  if (aggregation_it_ == aggregation_.end()) return false;

  // place aggregation values on the frame
  auto aggregation_values_it = aggregation_it_->second.values_.begin();
  for (const auto &aggregation_elem : self_.aggregations_)
    frame[aggregation_elem.output_sym] = *aggregation_values_it++;

  // place remember values on the frame
  auto remember_values_it = aggregation_it_->second.remember_.begin();
  for (const Symbol &remember_sym : self_.remember_)
    frame[remember_sym] = *remember_values_it++;

  aggregation_it_++;
  return true;
}

void Aggregate::AggregateCursor::ProcessAll(Frame &frame, Context &context) {
  ExpressionEvaluator evaluator(frame, context.parameters_,
                                context.symbol_table_, db_, GraphView::NEW);
  while (input_cursor_->Pull(frame, context))
    ProcessOne(frame, context.symbol_table_, evaluator);

  // calculate AVG aggregations (so far they have only been summed)
  for (int pos = 0; pos < static_cast<int>(self_.aggregations_.size()); ++pos) {
    if (self_.aggregations_[pos].op != Aggregation::Op::AVG) continue;
    for (auto &kv : aggregation_) {
      AggregationValue &agg_value = kv.second;
      int count = agg_value.counts_[pos];
      if (count > 0)
        agg_value.values_[pos] = agg_value.values_[pos] / (double)count;
    }
  }
}

void Aggregate::AggregateCursor::ProcessOne(Frame &frame,
                                            const SymbolTable &symbol_table,
                                            ExpressionEvaluator &evaluator) {
  std::vector<TypedValue> group_by;
  group_by.reserve(self_.group_by_.size());
  for (Expression *expression : self_.group_by_) {
    group_by.emplace_back(expression->Accept(evaluator));
  }

  AggregationValue &agg_value = aggregation_[group_by];
  EnsureInitialized(frame, agg_value);
  Update(frame, symbol_table, evaluator, agg_value);
}

void Aggregate::AggregateCursor::EnsureInitialized(
    Frame &frame,
    Aggregate::AggregateCursor::AggregationValue &agg_value) const {
  if (agg_value.values_.size() > 0) return;

  for (const auto &agg_elem : self_.aggregations_)
    agg_value.values_.emplace_back(DefaultAggregationOpValue(agg_elem));
  agg_value.counts_.resize(self_.aggregations_.size(), 0);

  for (const Symbol &remember_sym : self_.remember_)
    agg_value.remember_.push_back(frame[remember_sym]);
}

void Aggregate::AggregateCursor::Update(
    Frame &, const SymbolTable &, ExpressionEvaluator &evaluator,
    Aggregate::AggregateCursor::AggregationValue &agg_value) {
  DCHECK(self_.aggregations_.size() == agg_value.values_.size())
      << "Expected as much AggregationValue.values_ as there are "
         "aggregations.";
  DCHECK(self_.aggregations_.size() == agg_value.counts_.size())
      << "Expected as much AggregationValue.counts_ as there are "
         "aggregations.";

  // we iterate over counts, values and aggregation info at the same time
  auto count_it = agg_value.counts_.begin();
  auto value_it = agg_value.values_.begin();
  auto agg_elem_it = self_.aggregations_.begin();
  for (; count_it < agg_value.counts_.end();
       count_it++, value_it++, agg_elem_it++) {
    // COUNT(*) is the only case where input expression is optional
    // handle it here
    auto input_expr_ptr = agg_elem_it->value;
    if (!input_expr_ptr) {
      *count_it += 1;
      *value_it = *count_it;
      continue;
    }

    TypedValue input_value = input_expr_ptr->Accept(evaluator);

    // Aggregations skip Null input values.
    if (input_value.IsNull()) continue;
    const auto &agg_op = agg_elem_it->op;
    *count_it += 1;
    if (*count_it == 1) {
      // first value, nothing to aggregate. check type, set and continue.
      switch (agg_op) {
        case Aggregation::Op::MIN:
        case Aggregation::Op::MAX:
          *value_it = input_value;
          EnsureOkForMinMax(input_value);
          break;
        case Aggregation::Op::SUM:
        case Aggregation::Op::AVG:
          *value_it = input_value;
          EnsureOkForAvgSum(input_value);
          break;
        case Aggregation::Op::COUNT:
          *value_it = 1;
          break;
        case Aggregation::Op::COLLECT_LIST:
          value_it->Value<std::vector<TypedValue>>().push_back(input_value);
          break;
        case Aggregation::Op::COLLECT_MAP:
          auto key = agg_elem_it->key->Accept(evaluator);
          if (key.type() != TypedValue::Type::String)
            throw QueryRuntimeException("Map key must be a string");
          value_it->Value<std::map<std::string, TypedValue>>().emplace(
              key.Value<std::string>(), input_value);
          break;
      }
      continue;
    }

    // aggregation of existing values
    switch (agg_op) {
      case Aggregation::Op::COUNT:
        *value_it = *count_it;
        break;
      case Aggregation::Op::MIN: {
        EnsureOkForMinMax(input_value);
        try {
          TypedValue comparison_result = input_value < *value_it;
          // since we skip nulls we either have a valid comparison, or
          // an exception was just thrown above
          // safe to assume a bool TypedValue
          if (comparison_result.Value<bool>()) *value_it = input_value;
        } catch (const TypedValueException &) {
          throw QueryRuntimeException("Unable to get MIN of '{}' and '{}'",
                                      input_value.type(), value_it->type());
        }
        break;
      }
      case Aggregation::Op::MAX: {
        //  all comments as for Op::Min
        EnsureOkForMinMax(input_value);
        try {
          TypedValue comparison_result = input_value > *value_it;
          if (comparison_result.Value<bool>()) *value_it = input_value;
        } catch (const TypedValueException &) {
          throw QueryRuntimeException("Unable to get MAX of '{}' and '{}'",
                                      input_value.type(), value_it->type());
        }
        break;
      }
      case Aggregation::Op::AVG:
      // for averaging we sum first and divide by count once all
      // the input has been processed
      case Aggregation::Op::SUM:
        EnsureOkForAvgSum(input_value);
        *value_it = *value_it + input_value;
        break;
      case Aggregation::Op::COLLECT_LIST:
        value_it->Value<std::vector<TypedValue>>().push_back(input_value);
        break;
      case Aggregation::Op::COLLECT_MAP:
        auto key = agg_elem_it->key->Accept(evaluator);
        if (key.type() != TypedValue::Type::String)
          throw QueryRuntimeException("Map key must be a string");
        value_it->Value<std::map<std::string, TypedValue>>().emplace(
            key.Value<std::string>(), input_value);
        break;
    }  // end switch over Aggregation::Op enum
  }    // end loop over all aggregations
}

void Aggregate::AggregateCursor::Reset() {
  input_cursor_->Reset();
  aggregation_.clear();
  aggregation_it_ = aggregation_.begin();
  pulled_all_input_ = false;
}

void Aggregate::AggregateCursor::EnsureOkForMinMax(
    const TypedValue &value) const {
  switch (value.type()) {
    case TypedValue::Type::Bool:
    case TypedValue::Type::Int:
    case TypedValue::Type::Double:
    case TypedValue::Type::String:
      return;
    default:
      throw QueryRuntimeException(
          "Only Bool, Int, Double and String values are allowed in "
          "MIN and MAX aggregations");
  }
}
void Aggregate::AggregateCursor::EnsureOkForAvgSum(
    const TypedValue &value) const {
  switch (value.type()) {
    case TypedValue::Type::Int:
    case TypedValue::Type::Double:
      return;
    default:
      throw QueryRuntimeException(
          "Only numeric values allowed in SUM and AVG aggregations");
  }
}

bool TypedValueVectorEqual::operator()(
    const std::vector<TypedValue> &left,
    const std::vector<TypedValue> &right) const {
  DCHECK(left.size() == right.size())
      << "TypedValueVector comparison should only be done over vectors "
         "of the same size";
  return std::equal(left.begin(), left.end(), right.begin(),
                    TypedValue::BoolEqual{});
}

Skip::Skip(const std::shared_ptr<LogicalOperator> &input,
           Expression *expression)
    : input_(input), expression_(expression) {}

ACCEPT_WITH_INPUT(Skip)

std::unique_ptr<Cursor> Skip::MakeCursor(database::GraphDbAccessor &db) const {
  return std::make_unique<SkipCursor>(*this, db);
}

std::vector<Symbol> Skip::OutputSymbols(const SymbolTable &symbol_table) const {
  // Propagate this to potential Produce.
  return input_->OutputSymbols(symbol_table);
}

std::vector<Symbol> Skip::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

Skip::SkipCursor::SkipCursor(const Skip &self, database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

bool Skip::SkipCursor::Pull(Frame &frame, Context &context) {
  while (input_cursor_->Pull(frame, context)) {
    if (to_skip_ == -1) {
      // first successful pull from the input
      // evaluate the skip expression
      ExpressionEvaluator evaluator(frame, context.parameters_,
                                    context.symbol_table_, db_);
      TypedValue to_skip = self_.expression_->Accept(evaluator);
      if (to_skip.type() != TypedValue::Type::Int)
        throw QueryRuntimeException("Result of SKIP expression must be an int");

      to_skip_ = to_skip.Value<int64_t>();
      if (to_skip_ < 0)
        throw QueryRuntimeException(
            "Result of SKIP expression must be greater or equal to zero");
    }

    if (skipped_++ < to_skip_) continue;
    return true;
  }
  return false;
}

void Skip::SkipCursor::Reset() {
  input_cursor_->Reset();
  to_skip_ = -1;
  skipped_ = 0;
}

Limit::Limit(const std::shared_ptr<LogicalOperator> &input,
             Expression *expression)
    : input_(input), expression_(expression) {}

ACCEPT_WITH_INPUT(Limit)

std::unique_ptr<Cursor> Limit::MakeCursor(database::GraphDbAccessor &db) const {
  return std::make_unique<LimitCursor>(*this, db);
}

std::vector<Symbol> Limit::OutputSymbols(
    const SymbolTable &symbol_table) const {
  // Propagate this to potential Produce.
  return input_->OutputSymbols(symbol_table);
}

std::vector<Symbol> Limit::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

Limit::LimitCursor::LimitCursor(const Limit &self,
                                database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

bool Limit::LimitCursor::Pull(Frame &frame, Context &context) {
  // we need to evaluate the limit expression before the first input Pull
  // because it might be 0 and thereby we shouldn't Pull from input at all
  // we can do this before Pulling from the input because the limit
  // expression is not allowed to contain any identifiers
  if (limit_ == -1) {
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_);
    TypedValue limit = self_.expression_->Accept(evaluator);
    if (limit.type() != TypedValue::Type::Int)
      throw QueryRuntimeException("Result of LIMIT expression must be an int");

    limit_ = limit.Value<int64_t>();
    if (limit_ < 0)
      throw QueryRuntimeException(
          "Result of LIMIT expression must be greater or equal to zero");
  }

  // check we have not exceeded the limit before pulling
  if (pulled_++ >= limit_) return false;

  return input_cursor_->Pull(frame, context);
}

void Limit::LimitCursor::Reset() {
  input_cursor_->Reset();
  limit_ = -1;
  pulled_ = 0;
}

OrderBy::OrderBy(const std::shared_ptr<LogicalOperator> &input,
                 const std::vector<std::pair<Ordering, Expression *>> &order_by,
                 const std::vector<Symbol> &output_symbols)
    : input_(input), output_symbols_(output_symbols) {
  // split the order_by vector into two vectors of orderings and expressions
  std::vector<Ordering> ordering;
  ordering.reserve(order_by.size());
  order_by_.reserve(order_by.size());
  for (const auto &ordering_expression_pair : order_by) {
    ordering.emplace_back(ordering_expression_pair.first);
    order_by_.emplace_back(ordering_expression_pair.second);
  }
  compare_ = TypedValueVectorCompare(ordering);
}

ACCEPT_WITH_INPUT(OrderBy)

std::unique_ptr<Cursor> OrderBy::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<OrderByCursor>(*this, db);
}

std::vector<Symbol> OrderBy::OutputSymbols(
    const SymbolTable &symbol_table) const {
  // Propagate this to potential Produce.
  return input_->OutputSymbols(symbol_table);
}

std::vector<Symbol> OrderBy::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

OrderBy::OrderByCursor::OrderByCursor(const OrderBy &self,
                                      database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self_.input_->MakeCursor(db)) {}

bool OrderBy::OrderByCursor::Pull(Frame &frame, Context &context) {
  if (!did_pull_all_) {
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_);
    while (input_cursor_->Pull(frame, context)) {
      // collect the order_by elements
      std::vector<TypedValue> order_by;
      order_by.reserve(self_.order_by_.size());
      for (auto expression_ptr : self_.order_by_) {
        order_by.emplace_back(expression_ptr->Accept(evaluator));
      }

      // collect the output elements
      std::vector<TypedValue> output;
      output.reserve(self_.output_symbols_.size());
      for (const Symbol &output_sym : self_.output_symbols_)
        output.emplace_back(frame[output_sym]);

      cache_.emplace_back(std::move(order_by), std::move(output));
    }

    std::sort(cache_.begin(), cache_.end(),
              [this](const auto &pair1, const auto &pair2) {
                return self_.compare_(pair1.first, pair2.first);
              });

    did_pull_all_ = true;
    cache_it_ = cache_.begin();
  }

  if (cache_it_ == cache_.end()) return false;

  // place the output values on the frame
  DCHECK(self_.output_symbols_.size() == cache_it_->second.size())
      << "Number of values does not match the number of output symbols "
         "in OrderBy";
  auto output_sym_it = self_.output_symbols_.begin();
  for (const TypedValue &output : cache_it_->second)
    frame[*output_sym_it++] = output;

  cache_it_++;
  return true;
}

void OrderBy::OrderByCursor::Reset() {
  input_cursor_->Reset();
  did_pull_all_ = false;
  cache_.clear();
  cache_it_ = cache_.begin();
}

Merge::Merge(const std::shared_ptr<LogicalOperator> &input,
             const std::shared_ptr<LogicalOperator> &merge_match,
             const std::shared_ptr<LogicalOperator> &merge_create)
    : input_(input ? input : std::make_shared<Once>()),
      merge_match_(merge_match),
      merge_create_(merge_create) {}

bool Merge::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  if (visitor.PreVisit(*this)) {
    input_->Accept(visitor) && merge_match_->Accept(visitor) &&
        merge_create_->Accept(visitor);
  }
  return visitor.PostVisit(*this);
}

std::unique_ptr<Cursor> Merge::MakeCursor(database::GraphDbAccessor &db) const {
  return std::make_unique<MergeCursor>(*this, db);
}

std::vector<Symbol> Merge::ModifiedSymbols(const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  // Match and create branches should have the same symbols, so just take one of
  // them.
  auto my_symbols = merge_match_->OutputSymbols(table);
  symbols.insert(symbols.end(), my_symbols.begin(), my_symbols.end());
  return symbols;
}

Merge::MergeCursor::MergeCursor(const Merge &self,
                                database::GraphDbAccessor &db)
    : input_cursor_(self.input_->MakeCursor(db)),
      merge_match_cursor_(self.merge_match_->MakeCursor(db)),
      merge_create_cursor_(self.merge_create_->MakeCursor(db)) {}

bool Merge::MergeCursor::Pull(Frame &frame, Context &context) {
  if (pull_input_) {
    if (input_cursor_->Pull(frame, context)) {
      // after a successful input from the input
      // reset merge_match (it's expand iterators maintain state)
      // and merge_create (could have a Once at the beginning)
      merge_match_cursor_->Reset();
      merge_create_cursor_->Reset();
    } else
      // input is exhausted, we're done
      return false;
  }

  // pull from the merge_match cursor
  if (merge_match_cursor_->Pull(frame, context)) {
    // if successful, next Pull from this should not pull_input_
    pull_input_ = false;
    return true;
  } else {
    // failed to Pull from the merge_match cursor
    if (pull_input_) {
      // if we have just now pulled from the input
      // and failed to pull from merge_match, we should create
      __attribute__((unused)) bool merge_create_pull_result =
          merge_create_cursor_->Pull(frame, context);
      DCHECK(merge_create_pull_result) << "MergeCreate must never fail";
      return true;
    }
    // we have exhausted merge_match_cursor_ after 1 or more successful
    // Pulls
    // attempt next input_cursor_ pull
    pull_input_ = true;
    return Pull(frame, context);
  }
}

void Merge::MergeCursor::Reset() {
  input_cursor_->Reset();
  merge_match_cursor_->Reset();
  merge_create_cursor_->Reset();
  pull_input_ = true;
}

Optional::Optional(const std::shared_ptr<LogicalOperator> &input,
                   const std::shared_ptr<LogicalOperator> &optional,
                   const std::vector<Symbol> &optional_symbols)
    : input_(input ? input : std::make_shared<Once>()),
      optional_(optional),
      optional_symbols_(optional_symbols) {}

bool Optional::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  if (visitor.PreVisit(*this)) {
    input_->Accept(visitor) && optional_->Accept(visitor);
  }
  return visitor.PostVisit(*this);
}

std::unique_ptr<Cursor> Optional::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<OptionalCursor>(*this, db);
}

std::vector<Symbol> Optional::ModifiedSymbols(const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  auto my_symbols = optional_->ModifiedSymbols(table);
  symbols.insert(symbols.end(), my_symbols.begin(), my_symbols.end());
  return symbols;
}

Optional::OptionalCursor::OptionalCursor(const Optional &self,
                                         database::GraphDbAccessor &db)
    : self_(self),
      input_cursor_(self.input_->MakeCursor(db)),
      optional_cursor_(self.optional_->MakeCursor(db)) {}

bool Optional::OptionalCursor::Pull(Frame &frame, Context &context) {
  if (pull_input_) {
    if (input_cursor_->Pull(frame, context)) {
      // after a successful input from the input
      // reset optional_ (it's expand iterators maintain state)
      optional_cursor_->Reset();
    } else
      // input is exhausted, we're done
      return false;
  }

  // pull from the optional_ cursor
  if (optional_cursor_->Pull(frame, context)) {
    // if successful, next Pull from this should not pull_input_
    pull_input_ = false;
    return true;
  } else {
    // failed to Pull from the merge_match cursor
    if (pull_input_) {
      // if we have just now pulled from the input
      // and failed to pull from optional_ so set the
      // optional symbols to Null, ensure next time the
      // input gets pulled and return true
      for (const Symbol &sym : self_.optional_symbols_)
        frame[sym] = TypedValue::Null;
      pull_input_ = true;
      return true;
    }
    // we have exhausted optional_cursor_ after 1 or more successful Pulls
    // attempt next input_cursor_ pull
    pull_input_ = true;
    return Pull(frame, context);
  }
}

void Optional::OptionalCursor::Reset() {
  input_cursor_->Reset();
  optional_cursor_->Reset();
  pull_input_ = true;
}

Unwind::Unwind(const std::shared_ptr<LogicalOperator> &input,
               Expression *input_expression, Symbol output_symbol)
    : input_(input ? input : std::make_shared<Once>()),
      input_expression_(input_expression),
      output_symbol_(output_symbol) {}

ACCEPT_WITH_INPUT(Unwind)

std::unique_ptr<Cursor> Unwind::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<UnwindCursor>(*this, db);
}

std::vector<Symbol> Unwind::ModifiedSymbols(const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  symbols.emplace_back(output_symbol_);
  return symbols;
}

Unwind::UnwindCursor::UnwindCursor(const Unwind &self,
                                   database::GraphDbAccessor &db)
    : self_(self), db_(db), input_cursor_(self.input_->MakeCursor(db)) {}

bool Unwind::UnwindCursor::Pull(Frame &frame, Context &context) {
  if (db_.should_abort()) throw HintedAbortError();
  // if we reached the end of our list of values
  // pull from the input
  if (input_value_it_ == input_value_.end()) {
    if (!input_cursor_->Pull(frame, context)) return false;

    // successful pull from input, initialize value and iterator
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, db_);
    TypedValue input_value = self_.input_expression_->Accept(evaluator);
    if (input_value.type() != TypedValue::Type::List)
      throw QueryRuntimeException("UNWIND only accepts list values, got '{}'",
                                  input_value.type());
    input_value_ = input_value.Value<std::vector<TypedValue>>();
    input_value_it_ = input_value_.begin();
  }

  // if we reached the end of our list of values goto back to top
  if (input_value_it_ == input_value_.end()) return Pull(frame, context);

  frame[self_.output_symbol_] = *input_value_it_++;
  return true;
}

void Unwind::UnwindCursor::Reset() {
  input_cursor_->Reset();
  input_value_.clear();
  input_value_it_ = input_value_.end();
}

Distinct::Distinct(const std::shared_ptr<LogicalOperator> &input,
                   const std::vector<Symbol> &value_symbols)
    : input_(input ? input : std::make_shared<Once>()),
      value_symbols_(value_symbols) {}

ACCEPT_WITH_INPUT(Distinct)

std::unique_ptr<Cursor> Distinct::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<DistinctCursor>(*this, db);
}

std::vector<Symbol> Distinct::OutputSymbols(
    const SymbolTable &symbol_table) const {
  // Propagate this to potential Produce.
  return input_->OutputSymbols(symbol_table);
}

std::vector<Symbol> Distinct::ModifiedSymbols(const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

Distinct::DistinctCursor::DistinctCursor(const Distinct &self,
                                         database::GraphDbAccessor &db)
    : self_(self), input_cursor_(self.input_->MakeCursor(db)) {}

bool Distinct::DistinctCursor::Pull(Frame &frame, Context &context) {
  while (true) {
    if (!input_cursor_->Pull(frame, context)) return false;

    std::vector<TypedValue> row;
    row.reserve(self_.value_symbols_.size());
    for (const auto &symbol : self_.value_symbols_)
      row.emplace_back(frame[symbol]);
    if (seen_rows_.insert(std::move(row)).second) return true;
  }
}

void Distinct::DistinctCursor::Reset() {
  input_cursor_->Reset();
  seen_rows_.clear();
}

CreateIndex::CreateIndex(storage::Label label, storage::Property property)
    : label_(label), property_(property) {}

bool CreateIndex::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  return visitor.Visit(*this);
}

WITHOUT_SINGLE_INPUT(CreateIndex);

class CreateIndexCursor : public Cursor {
 public:
  CreateIndexCursor(const CreateIndex &self, database::GraphDbAccessor &db)
      : self_(self), db_(db) {}

  bool Pull(Frame &, Context &ctx) override {
    if (did_create_) return false;
    if (ctx.in_explicit_transaction_) {
      throw IndexInMulticommandTxException();
    }
    try {
      db_.BuildIndex(self_.label(), self_.property());
    } catch (const database::IndexExistsException &) {
      // Ignore creating an existing index.
    }
    ctx.is_index_created_ = did_create_ = true;
    return true;
  }

  void Reset() override { did_create_ = false; }

 private:
  const CreateIndex &self_;
  database::GraphDbAccessor &db_;
  bool did_create_ = false;
};

std::unique_ptr<Cursor> CreateIndex::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<CreateIndexCursor>(*this, db);
}

Union::Union(const std::shared_ptr<LogicalOperator> &left_op,
             const std::shared_ptr<LogicalOperator> &right_op,
             const std::vector<Symbol> &union_symbols,
             const std::vector<Symbol> &left_symbols,
             const std::vector<Symbol> &right_symbols)
    : left_op_(left_op),
      right_op_(right_op),
      union_symbols_(union_symbols),
      left_symbols_(left_symbols),
      right_symbols_(right_symbols) {}

std::unique_ptr<Cursor> Union::MakeCursor(database::GraphDbAccessor &db) const {
  return std::make_unique<Union::UnionCursor>(*this, db);
}

bool Union::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  if (visitor.PreVisit(*this)) {
    if (left_op_->Accept(visitor)) {
      right_op_->Accept(visitor);
    }
  }
  return visitor.PostVisit(*this);
}

std::vector<Symbol> Union::OutputSymbols(const SymbolTable &) const {
  return union_symbols_;
}

std::vector<Symbol> Union::ModifiedSymbols(const SymbolTable &) const {
  return union_symbols_;
}

WITHOUT_SINGLE_INPUT(Union);

Union::UnionCursor::UnionCursor(const Union &self,
                                database::GraphDbAccessor &db)
    : self_(self),
      left_cursor_(self.left_op_->MakeCursor(db)),
      right_cursor_(self.right_op_->MakeCursor(db)) {}

bool Union::UnionCursor::Pull(Frame &frame, Context &context) {
  std::unordered_map<std::string, TypedValue> results;
  if (left_cursor_->Pull(frame, context)) {
    // collect values from the left child
    for (const auto &output_symbol : self_.left_symbols_) {
      results[output_symbol.name()] = frame[output_symbol];
    }
  } else if (right_cursor_->Pull(frame, context)) {
    // collect values from the right child
    for (const auto &output_symbol : self_.right_symbols_) {
      results[output_symbol.name()] = frame[output_symbol];
    }
  } else {
    return false;
  }

  // put collected values on frame under union symbols
  for (const auto &symbol : self_.union_symbols_) {
    frame[symbol] = results[symbol.name()];
  }
  return true;
}

void Union::UnionCursor::Reset() {
  left_cursor_->Reset();
  right_cursor_->Reset();
}

bool PullRemote::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  if (visitor.PreVisit(*this)) {
    if (input_) input_->Accept(visitor);
  }
  return visitor.PostVisit(*this);
}

std::vector<Symbol> PullRemote::OutputSymbols(const SymbolTable &table) const {
  return input_ ? input_->OutputSymbols(table) : std::vector<Symbol>{};
}

std::vector<Symbol> PullRemote::ModifiedSymbols(
    const SymbolTable &table) const {
  auto symbols = symbols_;
  if (input_) {
    auto input_symbols = input_->ModifiedSymbols(table);
    symbols.insert(symbols.end(), input_symbols.begin(), input_symbols.end());
  }
  return symbols;
}

std::vector<Symbol> Synchronize::ModifiedSymbols(
    const SymbolTable &table) const {
  auto symbols = input_->ModifiedSymbols(table);
  if (pull_remote_) {
    auto pull_symbols = pull_remote_->ModifiedSymbols(table);
    symbols.insert(symbols.end(), pull_symbols.begin(), pull_symbols.end());
  }
  return symbols;
}

bool Synchronize::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  if (visitor.PreVisit(*this)) {
    input_->Accept(visitor) && pull_remote_->Accept(visitor);
  }
  return visitor.PostVisit(*this);
}

std::vector<Symbol> Cartesian::ModifiedSymbols(const SymbolTable &table) const {
  auto symbols = left_op_->ModifiedSymbols(table);
  auto right = right_op_->ModifiedSymbols(table);
  symbols.insert(symbols.end(), right.begin(), right.end());
  return symbols;
}

bool Cartesian::Accept(HierarchicalLogicalOperatorVisitor &visitor) {
  if (visitor.PreVisit(*this)) {
    left_op_->Accept(visitor) && right_op_->Accept(visitor);
  }
  return visitor.PostVisit(*this);
}

WITHOUT_SINGLE_INPUT(Cartesian);

PullRemoteOrderBy::PullRemoteOrderBy(
    const std::shared_ptr<LogicalOperator> &input, int64_t plan_id,
    const std::vector<std::pair<Ordering, Expression *>> &order_by,
    const std::vector<Symbol> &symbols)
    : input_(input), plan_id_(plan_id), symbols_(symbols) {
  CHECK(input_ != nullptr)
      << "PullRemoteOrderBy should always be constructed with input!";
  std::vector<Ordering> ordering;
  ordering.reserve(order_by.size());
  order_by_.reserve(order_by.size());
  for (const auto &ordering_expression_pair : order_by) {
    ordering.emplace_back(ordering_expression_pair.first);
    order_by_.emplace_back(ordering_expression_pair.second);
  }
  compare_ = TypedValueVectorCompare(ordering);
}

ACCEPT_WITH_INPUT(PullRemoteOrderBy);

std::vector<Symbol> PullRemoteOrderBy::OutputSymbols(
    const SymbolTable &table) const {
  return input_->OutputSymbols(table);
}

std::vector<Symbol> PullRemoteOrderBy::ModifiedSymbols(
    const SymbolTable &table) const {
  return input_->ModifiedSymbols(table);
}

namespace {

/** Helper class that wraps remote pulling for cursors that handle results from
 * distributed workers.
 */
class RemotePuller {
 public:
  RemotePuller(database::GraphDbAccessor &db,
               const std::vector<Symbol> &symbols, int64_t plan_id)
      : db_(db), symbols_(symbols), plan_id_(plan_id) {
    worker_ids_ = db_.db().remote_pull_clients().GetWorkerIds();
    // Remove master from the worker ids list.
    worker_ids_.erase(std::find(worker_ids_.begin(), worker_ids_.end(), 0));
  }

  void Initialize(Context &context) {
    if (!remote_pulls_initialized_) {
      for (auto &worker_id : worker_ids_) {
        UpdateRemotePullForWorker(worker_id, context);
      }
      remote_pulls_initialized_ = true;
    }
  }

  void Update(Context &context) {
    // If we don't have results for a worker, check if his remote pull
    // finished and save results locally.

    auto move_frames = [this](int worker_id, auto remote_results) {
      remote_results_[worker_id] = std::move(remote_results.frames);
      // Since we return and remove results from the back of the vector,
      // reverse the results so the first to return is on the end of the
      // vector.
      std::reverse(remote_results_[worker_id].begin(),
                   remote_results_[worker_id].end());
    };

    for (auto &worker_id : worker_ids_) {
      if (!remote_results_[worker_id].empty()) continue;

      auto found_it = remote_pulls_.find(worker_id);
      if (found_it == remote_pulls_.end()) continue;

      auto &remote_pull = found_it->second;
      if (!utils::IsFutureReady(remote_pull)) continue;

      auto remote_results = remote_pull.get();
      switch (remote_results.pull_state) {
        case distributed::RemotePullState::CURSOR_EXHAUSTED:
          move_frames(worker_id, remote_results);
          remote_pulls_.erase(found_it);
          break;
        case distributed::RemotePullState::CURSOR_IN_PROGRESS:
          move_frames(worker_id, remote_results);
          UpdateRemotePullForWorker(worker_id, context);
          break;
        case distributed::RemotePullState::SERIALIZATION_ERROR:
          throw mvcc::SerializationError(
              "Serialization error occured during PullRemote !");
        case distributed::RemotePullState::LOCK_TIMEOUT_ERROR:
          throw LockTimeoutException(
              "LockTimeout error occured during PullRemote !");
        case distributed::RemotePullState::UPDATE_DELETED_ERROR:
          throw QueryRuntimeException(
              "RecordDeleted error ocured during PullRemote !");
        case distributed::RemotePullState::RECONSTRUCTION_ERROR:
          throw query::ReconstructionException();
        case distributed::RemotePullState::QUERY_ERROR:
          throw QueryRuntimeException(
              "Query runtime error occurred duing PullRemote !");
      }
    }
  }

  auto Workers() { return worker_ids_; }

  int GetWorkerId(int worker_id_index) { return worker_ids_[worker_id_index]; }

  size_t WorkerCount() { return worker_ids_.size(); }

  void ClearWorkers() { worker_ids_.clear(); }

  bool HasPendingPulls() { return !remote_pulls_.empty(); }

  bool HasPendingPullFromWorker(int worker_id) {
    return remote_pulls_.find(worker_id) != remote_pulls_.end();
  }

  bool HasResultsFromWorker(int worker_id) {
    return !remote_results_[worker_id].empty();
  }

  std::vector<query::TypedValue> PopResultFromWorker(int worker_id) {
    auto result = remote_results_[worker_id].back();
    remote_results_[worker_id].pop_back();

    // Remove the worker if we exhausted all locally stored results and there
    // are no more pending remote pulls for that worker.
    if (remote_results_[worker_id].empty() &&
        remote_pulls_.find(worker_id) == remote_pulls_.end()) {
      worker_ids_.erase(
          std::find(worker_ids_.begin(), worker_ids_.end(), worker_id));
    }

    return result;
  }

 private:
  database::GraphDbAccessor &db_;
  std::vector<Symbol> symbols_;
  int64_t plan_id_;
  std::unordered_map<int, std::future<distributed::RemotePullData>>
      remote_pulls_;
  std::unordered_map<int, std::vector<std::vector<query::TypedValue>>>
      remote_results_;
  std::vector<int> worker_ids_;
  bool remote_pulls_initialized_ = false;

  void UpdateRemotePullForWorker(int worker_id, Context &context) {
    remote_pulls_[worker_id] = db_.db().remote_pull_clients().RemotePull(
        db_, worker_id, plan_id_, context.parameters_, symbols_, false);
  }
};

class PullRemoteCursor : public Cursor {
 public:
  PullRemoteCursor(const PullRemote &self, database::GraphDbAccessor &db)
      : self_(self),
        input_cursor_(self.input() ? self.input()->MakeCursor(db) : nullptr),
        remote_puller_(RemotePuller(db, self.symbols(), self.plan_id())) {}

  bool Pull(Frame &frame, Context &context) override {
    if (context.db_accessor_.should_abort()) throw HintedAbortError();
    remote_puller_.Initialize(context);

    bool have_remote_results = false;
    while (!have_remote_results && remote_puller_.WorkerCount() > 0) {
      if (context.db_accessor_.should_abort()) throw HintedAbortError();
      remote_puller_.Update(context);

      // Get locally stored results from workers in a round-robin fasion.
      int num_workers = remote_puller_.WorkerCount();
      for (int i = 0; i < num_workers; ++i) {
        int worker_id_index =
            (last_pulled_worker_id_index_ + i + 1) % num_workers;
        int worker_id = remote_puller_.GetWorkerId(worker_id_index);

        if (remote_puller_.HasResultsFromWorker(worker_id)) {
          last_pulled_worker_id_index_ = worker_id_index;
          have_remote_results = true;
          break;
        }
      }

      if (!have_remote_results) {
        if (!remote_puller_.HasPendingPulls()) {
          remote_puller_.ClearWorkers();
          break;
        }

        // If there are no remote results available, try to pull and return
        // local results.
        if (input_cursor_ && input_cursor_->Pull(frame, context)) {
          return true;
        }

        // If there aren't any local/remote results available, sleep.
        std::this_thread::sleep_for(
            std::chrono::microseconds(FLAGS_remote_pull_sleep_micros));
      }
    }

    // No more remote results, make sure local results get exhausted.
    if (!have_remote_results) {
      if (input_cursor_ && input_cursor_->Pull(frame, context)) {
        return true;
      }
      return false;
    }

    {
      int worker_id = remote_puller_.GetWorkerId(last_pulled_worker_id_index_);
      auto result = remote_puller_.PopResultFromWorker(worker_id);
      for (size_t i = 0; i < self_.symbols().size(); ++i) {
        frame[self_.symbols()[i]] = std::move(result[i]);
      }
    }
    return true;
  }

  void Reset() override {
    throw QueryRuntimeException("Unsupported: Reset during PullRemote!");
  }

 private:
  const PullRemote &self_;
  const std::unique_ptr<Cursor> input_cursor_;
  RemotePuller remote_puller_;
  int last_pulled_worker_id_index_ = 0;
};

class SynchronizeCursor : public Cursor {
 public:
  SynchronizeCursor(const Synchronize &self, database::GraphDbAccessor &db)
      : self_(self),
        input_cursor_(self.input()->MakeCursor(db)),
        pull_remote_cursor_(
            self.pull_remote() ? self.pull_remote()->MakeCursor(db) : nullptr) {
  }

  bool Pull(Frame &frame, Context &context) override {
    if (!initial_pull_done_) {
      InitialPull(frame, context);
      initial_pull_done_ = true;
    }
    // Yield local stuff while available.
    if (!local_frames_.empty()) {
      auto &result = local_frames_.back();
      for (size_t i = 0; i < frame.elems().size(); ++i) {
        if (self_.advance_command()) {
          query::ReconstructTypedValue(result[i]);
        }
        frame.elems()[i] = std::move(result[i]);
      }
      local_frames_.resize(local_frames_.size() - 1);
      return true;
    }

    // We're out of local stuff, yield from pull_remote if available.
    if (pull_remote_cursor_ && pull_remote_cursor_->Pull(frame, context))
      return true;

    return false;
  }

  void Reset() override {
    throw QueryRuntimeException("Unsupported: Reset during Synchronize!");
  }

 private:
  const Synchronize &self_;
  const std::unique_ptr<Cursor> input_cursor_;
  const std::unique_ptr<Cursor> pull_remote_cursor_;
  bool initial_pull_done_{false};
  std::vector<std::vector<TypedValue>> local_frames_;

  void InitialPull(Frame &frame, Context &context) {
    auto &db = context.db_accessor_.db();

    // Tell all workers to accumulate, only if there is a remote pull.
    std::vector<std::future<distributed::RemotePullData>> worker_accumulations;
    if (pull_remote_cursor_) {
      for (auto worker_id : db.remote_pull_clients().GetWorkerIds()) {
        if (worker_id == db.WorkerId()) continue;
        worker_accumulations.emplace_back(db.remote_pull_clients().RemotePull(
            context.db_accessor_, worker_id, self_.pull_remote()->plan_id(),
            context.parameters_, self_.pull_remote()->symbols(), true, 0));
      }
    }

    // Accumulate local results
    while (input_cursor_->Pull(frame, context)) {
      local_frames_.emplace_back();
      auto &local_frame = local_frames_.back();
      local_frame.reserve(frame.elems().size());
      for (auto &elem : frame.elems()) {
        local_frame.emplace_back(std::move(elem));
      }
    }

    // Wait for all workers to finish accumulation (first sync point).
    for (auto &accu : worker_accumulations) {
      switch (accu.get().pull_state) {
        case distributed::RemotePullState::CURSOR_EXHAUSTED:
          continue;
        case distributed::RemotePullState::CURSOR_IN_PROGRESS:
          throw QueryRuntimeException(
              "Expected exhausted cursor after remote pull accumulate");
        case distributed::RemotePullState::SERIALIZATION_ERROR:
          throw mvcc::SerializationError(
              "Failed to perform remote accumulate due to SerializationError");
        case distributed::RemotePullState::UPDATE_DELETED_ERROR:
          throw QueryRuntimeException(
              "Failed to perform remote accumulate due to RecordDeletedError");
        case distributed::RemotePullState::LOCK_TIMEOUT_ERROR:
          throw LockTimeoutException(
              "Failed to perform remote accumulate due to "
              "LockTimeoutException");
        case distributed::RemotePullState::RECONSTRUCTION_ERROR:
          throw QueryRuntimeException(
              "Failed to perform remote accumulate due to ReconstructionError");
        case distributed::RemotePullState::QUERY_ERROR:
          throw QueryRuntimeException(
              "Failed to perform remote accumulate due to Query runtime error");
      }
    }

    if (self_.advance_command()) {
      context.db_accessor_.AdvanceCommand();
    }

    // Make all the workers apply their deltas.
    auto tx_id = context.db_accessor_.transaction_id();
    auto apply_futures =
        db.remote_updates_clients().RemoteUpdateApplyAll(db.WorkerId(), tx_id);
    db.remote_updates_server().Apply(tx_id);
    for (auto &future : apply_futures) {
      switch (future.get()) {
        case distributed::RemoteUpdateResult::SERIALIZATION_ERROR:
          throw mvcc::SerializationError(
              "Failed to apply deferred updates due to SerializationError");
        case distributed::RemoteUpdateResult::UNABLE_TO_DELETE_VERTEX_ERROR:
          throw RemoveAttachedVertexException();
        case distributed::RemoteUpdateResult::UPDATE_DELETED_ERROR:
          throw QueryRuntimeException(
              "Failed to apply deferred updates due to RecordDeletedError");
        case distributed::RemoteUpdateResult::LOCK_TIMEOUT_ERROR:
          throw LockTimeoutException(
              "Failed to apply deferred update due to LockTimeoutException");
        case distributed::RemoteUpdateResult::DONE:
          break;
      }
    }

    // If the command advanced, let the workers know.
    if (self_.advance_command()) {
      auto futures =
          db.remote_pull_clients().NotifyAllTransactionCommandAdvanced(tx_id);
      for (auto &future : futures) future.wait();
    }
  }
};

class CartesianCursor : public Cursor {
 public:
  CartesianCursor(const Cartesian &self, database::GraphDbAccessor &db)
      : self_(self),
        left_op_cursor_(self.left_op()->MakeCursor(db)),
        right_op_cursor_(self_.right_op()->MakeCursor(db)) {
    CHECK(left_op_cursor_ != nullptr)
        << "CartesianCursor: Missing left operator cursor.";
    CHECK(right_op_cursor_ != nullptr)
        << "CartesianCursor: Missing right operator cursor.";
  }

  bool Pull(Frame &frame, Context &context) override {
    auto copy_frame = [&frame]() {
      std::vector<TypedValue> result;
      for (auto &elem : frame.elems()) {
        result.emplace_back(std::move(elem));
      }
      return result;
    };

    if (!cartesian_pull_initialized_) {
      // Pull all left_op frames.
      while (left_op_cursor_->Pull(frame, context)) {
        left_op_frames_.emplace_back(copy_frame());
      }

      // We're setting the iterator to 'end' here so it pulls the right cursor.
      left_op_frames_it_ = left_op_frames_.end();
      cartesian_pull_initialized_ = true;
    }

    // If left operator yielded zero results there is no cartesian product.
    if (left_op_frames_.empty()) {
      return false;
    }

    auto restore_frame = [&frame](const std::vector<Symbol> &symbols,
                                  const std::vector<TypedValue> &restore_from) {
      for (const auto &symbol : symbols) {
        frame[symbol] = restore_from[symbol.position()];
      }
    };

    if (left_op_frames_it_ == left_op_frames_.end()) {
      // Advance right_op_cursor_.
      if (!right_op_cursor_->Pull(frame, context)) return false;

      right_op_frame_ = copy_frame();
      left_op_frames_it_ = left_op_frames_.begin();
    } else {
      // Make sure right_op_cursor last pulled results are on frame.
      restore_frame(self_.right_symbols(), right_op_frame_);
    }

    restore_frame(self_.left_symbols(), *left_op_frames_it_);
    left_op_frames_it_++;
    return true;
  }

  void Reset() override {
    left_op_cursor_->Reset();
    right_op_cursor_->Reset();
    right_op_frame_.clear();
    left_op_frames_.clear();
    left_op_frames_it_ = left_op_frames_.end();
    cartesian_pull_initialized_ = false;
  }

 private:
  const Cartesian &self_;
  std::vector<std::vector<TypedValue>> left_op_frames_;
  std::vector<TypedValue> right_op_frame_;
  const std::unique_ptr<Cursor> left_op_cursor_;
  const std::unique_ptr<Cursor> right_op_cursor_;
  std::vector<std::vector<TypedValue>>::iterator left_op_frames_it_;
  bool cartesian_pull_initialized_{false};
};

class PullRemoteOrderByCursor : public Cursor {
 public:
  PullRemoteOrderByCursor(const PullRemoteOrderBy &self,
                          database::GraphDbAccessor &db)
      : self_(self),
        input_(self.input()->MakeCursor(db)),
        remote_puller_(RemotePuller(db, self.symbols(), self.plan_id())) {}

  bool Pull(Frame &frame, Context &context) {
    if (context.db_accessor_.should_abort()) throw HintedAbortError();
    ExpressionEvaluator evaluator(frame, context.parameters_,
                                  context.symbol_table_, context.db_accessor_);

    auto evaluate_result = [this, &evaluator]() {
      std::vector<TypedValue> order_by;
      order_by.reserve(self_.order_by().size());
      for (auto expression_ptr : self_.order_by()) {
        order_by.emplace_back(expression_ptr->Accept(evaluator));
      }
      return order_by;
    };

    auto restore_frame = [&frame,
                          this](const std::vector<TypedValue> &restore_from) {
      for (size_t i = 0; i < restore_from.size(); ++i) {
        frame[self_.symbols()[i]] = restore_from[i];
      }
    };

    if (!merge_initialized_) {
      remote_puller_.Initialize(context);
      missing_results_from_ = remote_puller_.Workers();
      missing_master_result_ = true;
      merge_initialized_ = true;
    }

    if (missing_master_result_) {
      if (input_->Pull(frame, context)) {
        std::vector<TypedValue> output;
        output.reserve(self_.symbols().size());
        for (const Symbol &symbol : self_.symbols()) {
          output.emplace_back(frame[symbol]);
        }

        merge_.push_back(MergeResultItem{std::experimental::nullopt, output,
                                         evaluate_result()});
      }
      missing_master_result_ = false;
    }

    while (!missing_results_from_.empty()) {
      if (context.db_accessor_.should_abort()) throw HintedAbortError();
      remote_puller_.Update(context);

      bool has_all_result = true;
      for (auto &worker_id : missing_results_from_) {
        if (!remote_puller_.HasResultsFromWorker(worker_id) &&
            remote_puller_.HasPendingPullFromWorker(worker_id)) {
          has_all_result = false;
          break;
        }
      }

      if (!has_all_result) {
        // If we don't have results from all workers, sleep before continuing.
        std::this_thread::sleep_for(
            std::chrono::microseconds(FLAGS_remote_pull_sleep_micros));
        continue;
      }

      for (auto &worker_id : missing_results_from_) {
        // It is possible that the workers remote pull finished but it didn't
        // return any results. In that case, just skip it.
        if (!remote_puller_.HasResultsFromWorker(worker_id)) continue;
        auto remote_result = remote_puller_.PopResultFromWorker(worker_id);
        restore_frame(remote_result);
        merge_.push_back(
            MergeResultItem{worker_id, remote_result, evaluate_result()});
      }

      missing_results_from_.clear();
    }

    if (merge_.empty()) return false;

    auto result_it = std::min_element(
        merge_.begin(), merge_.end(), [this](const auto &lhs, const auto &rhs) {
          return self_.compare()(lhs.order_by, rhs.order_by);
        });

    restore_frame(result_it->remote_result);

    if (result_it->worker_id) {
      missing_results_from_.push_back(result_it->worker_id.value());
    } else {
      missing_master_result_ = true;
    }

    merge_.erase(result_it);
    return true;
  }

  void Reset() {
    throw QueryRuntimeException("Unsupported: Reset during PullRemoteOrderBy!");
  }

 private:
  struct MergeResultItem {
    std::experimental::optional<int> worker_id;
    std::vector<TypedValue> remote_result;
    std::vector<TypedValue> order_by;
  };

  const PullRemoteOrderBy &self_;
  std::unique_ptr<Cursor> input_;
  RemotePuller remote_puller_;
  std::vector<MergeResultItem> merge_;
  std::vector<int> missing_results_from_;
  bool missing_master_result_ = false;
  bool merge_initialized_ = false;
};

}  // namespace

std::unique_ptr<Cursor> PullRemote::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<PullRemoteCursor>(*this, db);
}

std::unique_ptr<Cursor> Synchronize::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<SynchronizeCursor>(*this, db);
}

std::unique_ptr<Cursor> Cartesian::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<CartesianCursor>(*this, db);
}

std::unique_ptr<Cursor> PullRemoteOrderBy::MakeCursor(
    database::GraphDbAccessor &db) const {
  return std::make_unique<PullRemoteOrderByCursor>(*this, db);
}

}  // namespace query::plan

BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Once);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::CreateNode);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::CreateExpand);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ScanAll);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ScanAllByLabel);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ScanAllByLabelPropertyRange);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ScanAllByLabelPropertyValue);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Expand);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ExpandVariable);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Filter);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Produce);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ConstructNamedPath);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Delete);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::SetProperty);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::SetProperties);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::SetLabels);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::RemoveProperty);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::RemoveLabels);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::ExpandUniquenessFilter<EdgeAccessor>);
BOOST_CLASS_EXPORT_IMPLEMENT(
    query::plan::ExpandUniquenessFilter<VertexAccessor>);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Accumulate);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Aggregate);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Skip);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Limit);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::OrderBy);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Merge);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Optional);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Unwind);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Distinct);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::CreateIndex);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Union);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::PullRemote);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Synchronize);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::Cartesian);
BOOST_CLASS_EXPORT_IMPLEMENT(query::plan::PullRemoteOrderBy);