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 and Syntactic Analysis
Semantic Analysis and Symbol Generation
Graph Pattern Matching Semantics
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.