From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | "Peter J(dot) Holzer" <hjp-pgsql(at)hjp(dot)at> |
Cc: | pgsql-general(at)lists(dot)postgresql(dot)org |
Subject: | Re: Lots of read activity on index only scan |
Date: | 2022-11-18 20:59:46 |
Message-ID: | 2267925.1668805186@sss.pgh.pa.us |
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Lists: | pgsql-general |
"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. Did you vacuum this table after building it, or wait
long enough for autovacuum to do so?
regards, tom lane
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