Buffers & memorysenior🔒 Pro concept

BufferAccessStrategy ring buffers: why big seq scans/VACUUM/COPY don't trash shared_buffers

Simple terms

One big scan over a huge table could shove everybody else's cached pages out of memory. Postgres doesn't let it. A large sequential scan gets handed a small pool of buffers and told to reuse them, round and round, instead of claiming fresh cache for every page it touches. The scan still runs at full speed. It just leaves barely a dent, and the working set everyone else depends on is still sitting there warm when it finishes.

You might be asked

Why doesn't a large sequential scan (or VACUUM, or COPY) evict your entire shared_buffers cache? Explain BufferAccessStrategy ring buffers, when they kick in, how big they are, and how you'd prove it.

Topicbuffer manager / shared_buffers / IO strategy
PostgreSQL14, 15, 16, 17, 18
Tools usedDocker pgi17 (PostgreSQL 17.10), pg_buffercache + pg_buffercache_evict, source-cache @REL_17_10 (heapam.c, freelist.c)
Last reviewed2026-06-19

Pro concept

Full answer and evidence depth sit behind Pro

You have the question and a plain-English lead. Pro unlocks the short answer, what the docs say, what the code does, the labeled lab evidence or reproduction protocol, how to use it under pressure, and the references.

ProFull answer + evidence depth

Unlock the full breakdown for BufferAccessStrategy ring buffers: why big seq scans/VACUUM/COPY don't trash shared_buffers

You have the interview question and a plain-English lead. Pro opens the short answer, the manual walkthrough, the source decode, and a 1-line evidence section that states whether it is raw output, a captured run summary, or a protocol to run yourself, plus how to use it under pressure.
  • How you'd answer it, the full senior-level short answer
  • What the docs say, the manual's actual wording, with the citations
  • What the code does, the mechanism decoded from PostgreSQL's own source at a pinned tag
  • Proof from a real run, labeled as raw output, a captured run summary, or a run-it-yourself protocol
  • Using it under pressure, the situation, the call you'd make, what goes wrong, and what people get wrong
  • Version notes, what changed across PostgreSQL releases

Card required. Cancel before day 7 and you are not charged.

Compare plans

How this was verified

The open teaser is the question and a plain-English lead. The short answer, the manual walkthrough, the source decode, the lab evidence, and how to use it under pressure unlock with Pro. Evidence is labeled as raw output, a captured run summary, or a run-it-yourself protocol.

Connected

Where this concept connects

How this concept links across the library, the interview questions that test it, its plain-English glossary definition, and the guided pathways it belongs to. Open the full map to explore further.

Open in the interactive map →
← All interview concepts