Re: Lots of read activity on index only scan

On 2022-11-18 15:59:46 -0500, Tom Lane wrote:
> "Peter J. Holzer" <hjp-pgsql(at)hjp(dot)at> writes:
> > Both do a parallel index only scan. Both perform 0 heap fetches.
> > But one reads 27336 buffers (or about 22 bytes per index entry, which
> > sounds reasonable) while the other reads 9995216 buffers (or almost one
> > full buffer per row). Why? The entries should be dense in the index in
> > both cases and since it's an index only scan (and explain says there
> > were 0 heap fetches) I would not expect extra accesses. Where do these
> > buffer reads come from?
>
> An "index only" scan is only that if the table's all-visible map is
> fully set.

Yes, but shouldn't that be reflected in the heap fetches? Or am I
misunderstanding what that means?

Plus:

> Did you vacuum this table after building it, or wait long enough for
> autovacuum to do so?

Both actually. I built the table this morning and did all the tests
again just before posting. And I explicitely vacuumed it during this
second set of tests. I should have mentioned that, sorry. Also possibly
relevant: This is on Pg 15.1 (PGDG build for Ubuntu).

hp

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