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List:       pgsql-hackers
Subject:    Re: [HACKERS] gincostestimate and hypothetical indexes
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2015-11-30 23:37:48
Message-ID: 28068.1448926668 () sss ! pgh ! pa ! us
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Julien Rouhaud <julien.rouhaud@dalibo.com> writes:
> I figured out that it's not possible to use a hypothetical gin index, as
> the gincostestimate function try to retrieve some statistical data from
> the index meta page.

Good point.

> Attached patch fixes this. I believe this should be back-patched as was
> a2095f7fb5a57ea1794f25d029756d9a140fd429.

I don't much care for this patch though.  The core problem is that just
returning all zeroes seems quite useless: it will probably result in silly
cost estimates.  The comment in the patch claiming that this would be the
situation in a never-vacuumed index is wrong, because ginbuild() updates
those stats too.  But I'm not sure exactly what to do instead :-(.

Ideally we'd put it on the head of the hypothetical-index plugin to invent
some numbers, but I dunno if we want to create such an API or not ... and
we certainly couldn't back-patch such a change.

Maybe we could do something along the lines of pretending that 90% of the
index size given by the plugin is entry pages?  Don't know what a good
ratio would be exactly, but we could probably come up with one with a bit
of testing.

			regards, tom lane


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