b01cbe12b3
Summary: This is a short update which should explain the primary entrypoint to query parsing and execution. Reviewers: mtomic, llugovic Reviewed By: mtomic Subscribers: pullbot Differential Revision: https://phabricator.memgraph.io/D1856
35 lines
1.6 KiB
Markdown
35 lines
1.6 KiB
Markdown
# Query Parsing, Planning and Execution
|
|
|
|
This part of the documentation deals with query execution.
|
|
|
|
Memgraph currently supports only query interpretation. Each new query is
|
|
parsed, analysed and translated into a sequence of operations which are then
|
|
executed on the main database storage. Query execution is organized into the
|
|
following phases:
|
|
|
|
1. [Lexical Analysis (Tokenization)](parsing.md)
|
|
2. [Syntactic Analysis (Parsing)](parsing.md)
|
|
3. [Semantic Analysis and Symbol Generation](semantic.md)
|
|
4. [Logical Planning](planning.md)
|
|
5. [Logical Plan Execution](execution.md)
|
|
|
|
The main entry point is `Interpreter::operator()`, which takes a query text
|
|
string and produces a `Results` object. To instantiate the object,
|
|
`Interpreter` needs to perform the above steps from 1 to 4. If any of the
|
|
steps fail, a `QueryException` is thrown. The complete `LogicalPlan` is
|
|
wrapped into a `CachedPlan` and stored for reuse. This way we can skip the
|
|
whole process of analysing a query if it appears to be the same as before.
|
|
|
|
When we have valid plan, the client code can invoke `Results::PullAll` with a
|
|
stream object. The `Results` instance will then execute the plan and fill the
|
|
stream with the obtained results.
|
|
|
|
Since we want to optionally run Memgraph as a distributed database, we have
|
|
hooks for creating a different plan of logical operators.
|
|
`DistributedInterpreter` inherits from `Interpreter` and overrides
|
|
`MakeLogicalPlan` method. This method needs to return a concrete instance of
|
|
`LogicalPlan`, and in case of distributed database that will be
|
|
`DistributedLogicalPlan`.
|
|
|
|
|