Statistics and Selectivity: Why the Planner Guesses Wrong
To pick a plan, the planner has to guess how many rows each step will return.
Those guesses come from a sample of your data that ANALYZE collects and stores. When that sample is stale or missing, the guesses go wrong, the planner picks a bad plan, and a query that should be fast crawls.
Pro members see the rest of this lesson
- A 4-step state diagram: Collect → Estimate → Multiply → Correct
- The full mechanism: what PostgreSQL does internally, in source terms
- 3 SQL queries you can run, each labelled by how it was verified
- 1 transcript captured on PostgreSQL 17.10
- The closing insight, the mistake it prevents, and what it changes in your work
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Check it against the source3 citations in postgres/postgres · file, symbol and line verified on REL_17_STABLE
Primary symbol: analyze_rel · line 111
Primary symbol: eqsel · line 228
Primary symbol: clauselist_selectivity · line 100
Anchored to postgres/postgres on REL_17_STABLE and cross-checked against the manual for PostgreSQL 15–18. Every query here was run and its output captured on a throwaway PostgreSQL 17.10 lab; corrections are noted inline. §14.2 Statistics Used by the Planner in the official docs →
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