Bitmap Scans: How PostgreSQL Combines Indexes
Reading an index is best when you want just a few rows, and reading the whole table is best when you want most of them.
For the awkward middle, PostgreSQL builds a bitmap: it first collects which pages hold matching rows, then visits those pages in order. It can even merge several indexes this way.
Pro members see the rest of this lesson
- A 4-step state diagram: Scan → Combine → Degrade → Fetch
- 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 source1 citation in postgres/postgres · file, symbol and line verified on REL_17_STABLE
Primary symbol: BitmapHeapNext · line 69
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. §14.1 Using EXPLAIN (bitmap scans) in the official docs →
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