Query planner
A query planner is a database component that evaluates possible execution strategies and constructs or selects a plan for a query.
100 SQL Server Mistakes and How to Avoid Them
A query optimizer selects an efficient execution plan for a declarative database query.
- How it is used
- Determines efficient execution plans for declarative T-SQL and guides performance troubleshooting when selected plans are poor.
- How deep it goes
- Explains parsing, algebraization, metadata, costs, and physical joins, but does not detail the optimizer’s full algorithm.
- Examples
- Uses an impressions join, execution-plan figures, join hints, and indexes on join columns to illustrate plan selection.
- What you can do after
- Diagnose/debug with it by examining plans, questioning forced operators, and checking indexes and statistics before applying hints.
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PostgreSQL Mistakes and How to Avoid Them
A query planner or optimizer is the database component that selects and may transform an execution strategy for a SQL query.
- How it is used
- Diagnoses SQL performance and explains why query structure, statistics, and configuration influence execution strategy.
- How deep it goes
- Explains cost-based plan selection, statistics, filter reordering, CTE inlining, and actual timing, but not detailed plan-node internals.
- Examples
- Uses unpaid-invoice email SQL, paired non-CTE and CTE queries, their plans, and EXPLAIN ANALYZE timing comparisons.
- What you can do after
- Diagnose/debug with EXPLAIN and EXPLAIN ANALYZE, then restructure SQL and maintain statistics to improve plan quality.
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Related concepts
- execution planproduces · An execution plan is a structured description of the operations or tasks arranged to carry out a query or other task.
- indexinguses · Indexing is the organization or use of positions, keys, or lookup structures to identify and retrieve selected data or elements.
- statisticsuses · Statistics is the discipline and set of methods for collecting, summarizing, analyzing, and interpreting data, including numerical descriptions used to characterize observations and support inference.
- hidden indexcontrasts with · A hidden index is an index configured to be omitted from certain ordinary discovery, listing, planning, or selection operations.
- composite indexuses · A composite index is a database index whose key contains multiple fields or columns, ordered according to the index definition.
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