The Planner: Generating the Optimal Query Plan
The planner acts as the brain behind query optimization in PostgreSQL. It receives a query tree from the rewriter, which represents the parsed and normalized query, and its primary objective is to generate a query plan tree that can be executed most efficiently by the executor. Unlike rule-based optimization or hints, PostgreSQL's planner relies solely on cost-based optimization strategies.
Composing the Plan Tree: Plan Nodes and Structure
The plan tree is the result of the planner's work and consists of plan nodes, which are elements defining various operations and steps involved in query execution. These plan nodes hold information required by the executor for processing. The plan tree, connected to the PlannedStmt structure, is represented as a list of plan nodes defined in plannodes.h. For a more detailed understanding of the plan nodes, we refer to Section 3.3.3 and Section 3.5.4.2 of the PostgreSQL documentation.
Executor: Bringing the Plan to Life
The executor is responsible for executing the query plan generated by the planner. It operates on the plan tree, starting from the end and working its way up to the root. In the case of a single-table query, the executor processes the plan nodes accordingly, leveraging the information provided by each node. For instance, if the plan tree includes a sort node followed by a sequential scan node, the executor will first perform a sequential scan on the table and then sort the obtained result.
Interacting with the Database Cluster: Reading, Writing, and Memory Management
To fulfill its tasks, the executor interacts with the database cluster, reading and writing tables and indexes via the buffer manager, as described in Chapter 8 of the PostgreSQL documentation. During query processing, the executor utilizes allocated memory areas such as temp_buffers and work_mem, which are pre-allocated for efficient memory management. Additionally, the executor may create temporary files if necessary to handle intermediate results during execution.
The planner and executor are fundamental components of PostgreSQL's query processing system. While the planner generates the optimal query plan based on cost-based optimization, the executor brings that plan to life, executing the operations defined by the plan nodes. By understanding the intricacies of these components and their interactions, developers and database administrators can fine-tune query performance, optimize resource usage, and ultimately improve the efficiency of their PostgreSQL databases.
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