| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, Hadi Moshayedi <hadi(at)citusdata(dot)com>, Postgres General <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: index-only-scan when there is an index on all columns |
| Date: | 2018-02-27 22:11:41 |
| Message-ID: | 20180227221141.tubymz7ol4wjoqiz@alap3.anarazel.de |
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| Lists: | pgsql-general |
On 2018-02-27 16:58:11 -0500, Tom Lane wrote:
> > Also, I recall discussion that select statements will touch the visibility
> > map (hence causing write I/O even in a read-only query) but [1] indicates
> > that only vacuum will set them ddl will clear them.
>
> Hm, I don't recall that, but I've not been involved in the last few rounds
> of hacking on that mechanism.
I'm fairly certain that only vacuum and table rewrites like cluster sets
all-visible, and that there was never any released code that did so
during SELECT. I think there were a few patches debating whether we
could change that, but they never really got anywhere afair.
Greetings,
Andres Freund
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