SQL reaches the database
An application sends a SQL query over its database connection. The server receives a description of the result the application wants.
How PostgreSQL executes a query
The parser, planner, and executor behind a SQL query.
Process overview
- SQL
- Parse
- Rewrite
- Plan
- Execute
Steps
5 stepsText becomes a query tree
PostgreSQL checks syntax and transforms the query into a tree. Names and expressions gain meaning through the database’s catalog.
Rules reshape the query
The rewrite stage applies stored rules. A reference to a view can become a query against the base tables defined by that view.
The planner chooses a route
The planner considers possible access paths and estimates their costs. It selects an execution plan, which might use an index or scan a table.
The plan produces rows
The executor follows the plan, fetching rows and applying operations such as joins, filters, and sorts. The resulting rows return to the application.
Keyboard: ←→
Scope
The PostgreSQL query pipeline, simplified. Plan choices depend on statistics, indexes, configuration, and the actual query.
