| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
| Cc: | 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 21:58:11 |
| Message-ID: | 28385.1519768691@sss.pgh.pa.us |
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| Lists: | pgsql-general |
"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> If one runs vacuum on a table (small or otherwise) that is currently
> choosing an index scan as its best plan how likely is it that post-vacuum
> an index-only plan would be chosen if the index type and column presence
> conditions are met?
Offhand I think it would always prefer IOS over regular indexscan if the
table is mostly all-visible. The problem in this example was that other
choices dominate both.
> 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.
regards, tom lane
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