| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Lukas Fittl <lukas(at)fittl(dot)com> |
| Cc: | Tomas Vondra <tomas(at)vondra(dot)me>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: EXPLAIN: showing ReadStream / prefetch stats |
| Date: | 2026-04-07 15:09:40 |
| Message-ID: | sr5bcs5g73675iqxqc7g62yud6jplcyayzdeumh6ne5qvk64iu@gct7k5v2upnb |
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| Lists: | pgsql-hackers |
Hi,
On 2026-04-07 01:00:28 -0700, Lukas Fittl wrote:
> Btw, in that same test, when I re-run I get Prefetch but not I/O -- why is that?
>
> Seq Scan on organizations (cost=0.00..7.42 rows=1 width=483) (actual
> time=0.034..0.042 rows=1.00 loops=1)
> Filter: (slug = 'pganalyze'::text)
> Rows Removed by Filter: 113
> Prefetch: avg=1.00 max=1 capacity=94
> Buffers: shared hit=6
>
> (if its all buffers hit, why are there any prefetches at all?)
It's not really prefetching, which is visible as the distance being 1. The
read stream is created at the start of the scan, not at the first miss.
Greetings,
Andres Freund
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