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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
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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:
- Lexical Analysis (Tokenization)
- Syntactic Analysis (Parsing)
- Semantic Analysis and Symbol Generation
- Logical Planning
- Logical Plan Execution
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.
