Buffer Cache: How shared_buffers and Clock-Sweep Work
PostgreSQL can't work on a page while it's still on disk, so it copies pages into a shared pool of memory slots called shared_buffers first.
Every query reads and writes through that pool. When the pool fills up, the database has to evict an old page, and clock-sweep is how it picks which one.
Pro members see the rest of this lesson
- A 4-step state diagram: Resident → Sweep → Victim → Reuse
- The full mechanism: what PostgreSQL does internally, in source terms
- 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: BufferDesc · line 245
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. §20.4 Resource Consumption (Memory) in the official docs →
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