plan::OrderBy - use vector instead of list

Summary: Reduces latency on LDBC query 9 from 7.9sec to 6.8sec (14%). That query has 650k rows in ORDER BY, 3 ordering elements and 10ish values get returned (both of them are now accumulated into vectors).

Reviewers: buda, mislav.bradac

Reviewed By: mislav.bradac

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D775

src/query/plan/operator.cpp

@@ -1,6 +1,7 @@
 #include <algorithm>
 #include <limits>
 #include <type_traits>
+#include <utility>
 
 #include "query/plan/operator.hpp"
 
@@ -2018,7 +2019,7 @@ OrderBy::OrderBy(const std::shared_ptr<LogicalOperator> &input,
     ordering.emplace_back(ordering_expression_pair.first);
     order_by_.emplace_back(ordering_expression_pair.second);
   }
-  compare_ = TypedValueListCompare(ordering);
+  compare_ = TypedValueVectorCompare(ordering);
 }
 
 ACCEPT_WITH_INPUT(OrderBy)
@@ -2041,17 +2042,19 @@ bool OrderBy::OrderByCursor::Pull(Frame &frame,
     ExpressionEvaluator evaluator(frame, symbol_table, db_);
     while (input_cursor_->Pull(frame, symbol_table)) {
       // collect the order_by elements
-      std::list<TypedValue> order_by;
+      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::list<TypedValue> output;
+      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(order_by, output);
+      cache_.emplace_back(std::move(order_by), std::move(output));
     }
 
     std::sort(cache_.begin(), cache_.end(),
@@ -2126,16 +2129,16 @@ bool OrderBy::TypedValueCompare(const TypedValue &a, const TypedValue &b) {
   }
 }
 
-bool OrderBy::TypedValueListCompare::operator()(
+bool OrderBy::TypedValueVectorCompare::operator()(
-    const std::list<TypedValue> &c1, const std::list<TypedValue> &c2) const {
+    const std::vector<TypedValue> &c1,
-  auto c1_it = c1.begin();
+    const std::vector<TypedValue> &c2) const {
-  auto c2_it = c2.begin();
   // ordering is invalid if there are more elements in the collections
   // then there are in the ordering_ vector
-  debug_assert(std::distance(c1_it, c1.end()) <= ordering_.size() &&
+  debug_assert(c1.size() <= ordering_.size() && c2.size() <= ordering_.size(),
-                   std::distance(c2_it, c2.end()) <= ordering_.size(),
                "Collections contain more elements then there are orderings");
 
+  auto c1_it = c1.begin();
+  auto c2_it = c2.begin();
   auto ordering_it = ordering_.begin();
   for (; c1_it != c1.end() && c2_it != c2.end();
        c1_it++, c2_it++, ordering_it++) {

src/query/plan/operator.hpp

@@ -1351,24 +1351,24 @@ class OrderBy : public LogicalOperator {
   const auto &output_symbols() const { return output_symbols_; }
 
  private:
-  // custom Comparator type for comparing lists of TypedValues
+  // custom Comparator type for comparing vectors of TypedValues
   // does lexicographical ordering of elements based on the above
   // defined TypedValueCompare, and also accepts a vector of Orderings
   // the define how respective elements compare
-  class TypedValueListCompare {
+  class TypedValueVectorCompare {
    public:
-    TypedValueListCompare() {}
+    TypedValueVectorCompare() {}
-    TypedValueListCompare(const std::vector<Ordering> &ordering)
+    TypedValueVectorCompare(const std::vector<Ordering> &ordering)
         : ordering_(ordering) {}
-    bool operator()(const std::list<TypedValue> &c1,
+    bool operator()(const std::vector<TypedValue> &c1,
-                    const std::list<TypedValue> &c2) const;
+                    const std::vector<TypedValue> &c2) const;
 
    private:
     std::vector<Ordering> ordering_;
   };
 
   const std::shared_ptr<LogicalOperator> input_;
-  TypedValueListCompare compare_;
+  TypedValueVectorCompare compare_;
   std::vector<Expression *> order_by_;
   const std::vector<Symbol> output_symbols_;
 
@@ -1394,7 +1394,7 @@ class OrderBy : public LogicalOperator {
     // first pair element is the order-by list
     // second pair is the remember list
     // the cache is filled and sorted (only on first pair elem) on first Pull
-    std::vector<std::pair<std::list<TypedValue>, std::list<TypedValue>>> cache_;
+    std::vector<std::pair<std::vector<TypedValue>, std::vector<TypedValue>>> cache_;
     // iterator over the cache_, maintains state between Pulls
     decltype(cache_.begin()) cache_it_ = cache_.begin();
   };