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PostgreSQL separates static query rewrites from data-driven plan optimization

theconsensus.dev by theconsensus.dev via fanf 14 min read Editor score 7/10

The piece distinguishes statically knowable rewrites, such as folding `i+0`, from optimization decisions that depend on statistics and data. Understanding that boundary helps engineers reason about query plans and database performance.

What: - PostgreSQL query-plan rewriting covers statically knowable changes, distinct from optimization that depends on statistics or other data-specific knowledge.
How: - PostgreSQL parses SQL into an abstract syntax tree, then resolves identifiers; its planner can fold constants and choose indexes, scans, and joins.
Why it matters: - Distinguishing rewriting from optimization helps engineers reason about which query-plan changes follow from SQL alone and which depend on data.

PostgreSQL turns SQL into an AST, then the planner produces a query plan. Sketched from the article's own text.

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