| From: | Nikolay Samokhvalov <nik(at)postgres(dot)ai> |
|---|---|
| To: | Tomas Vondra <tomas(at)vondra(dot)me> |
| Cc: | PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: EXPLAIN: showing ReadStream / prefetch stats |
| Date: | 2026-09-22 19:02:56 |
| Message-ID: | CAM527d9dSPTBHEynuLH3bRR=hNb-Pj8RP=nh+LdO49ndcVsZ2A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Sep 22, 2026 at 8:23 AM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
> It's not clear to me why this would be desirable. The query execution
> clearly *does* perform the I/O, even if the buffers end up not being
> used. Hiding that would be very confusing / misleading.
>
> In other words, I/O and buffers may disagree - that's not a bug. Those
> counters are tracking different things.
Thanks all. Agreed that issued I/O should remain accounted for.
Is Prefetch avg/max also intended to include buffers discarded by
read_stream_reset()? If so, agreed, the patch is wrong. The comment above
read_stream_count_prefetch() says it is called once per buffer returned
to the consumer.
Nik
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