Execution Engine

Once a query has been parsed, planned, and optimized, the resulting PhysicalPlan is handed to the Execution Engine. This component is the workhorse of the database, responsible for actually running the plan and producing the final results.

Core Concepts

• Volcano (Iterator) Model: QuillSQL uses a pull-based execution model. Each operator in the physical plan tree is an "iterator" that provides a next() method. The parent operator calls next() on its children to pull rows upwards through the tree, from the storage layer to the client.

• Physical Operators: Each logical operation (like a filter or a join) is mapped to a concrete physical operator that implements a specific algorithm (e.g., PhysicalFilter, PhysicalNestedLoopJoin).

• Execution Context: As the plan is executed, a shared ExecutionContext is passed between operators. It contains vital information, such as the current transaction and its MVCC snapshot, allowing operators to perform visibility checks and locking.

This section contains:

• The Volcano Execution Model: A deep dive into the pull-based execution model and the lifecycle of a query.