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Re: Index-only scan performance regression

From: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Index-only scan performance regression
Date: 2012-02-01 09:09:56
Message-ID: CAEZATCVenh5+N=BeksYTacC5G+GhiCJnrgKny9opQin0OwSZHQ@mail.gmail.com (view raw or flat)
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On 31 January 2012 23:04, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> writes:
>> The thing I'm unsure about is whether sending sinval
>> messages when the visibility map is extended is a good idea.
>
> Seems perfectly reasonable to me.  They'd occur so seldom as to be
> more than repaid if we can scrape some cost out of the mainline paths.
>

OK, thanks. That's good.


> The real objection to this probably is that if it only saves anything
> for tables that don't have a VM yet, it's dubious whether it's worth
> doing.  But if we can avoid wasted checks for VM extension as well,
> then I think it's probably a no-brainer.
>
>                        regards, tom lane

Yes it applies in the same way to VM extension - if the table has
grown and the VM has not yet been extended, but I don't see why that
is any worse than the case of not having a VM yet.

Actually I think that this is not such an uncommon case - for a table
which has only had data inserted - no deletes or updates - it is
tempting to think that ANALYSE is sufficient, and that there is no
need to VACUUM. If it were simply the case that this caused an
index-only scan to have no real benefit, you might be willing to live
with normal index scan performance. But actually it causes a very
significant performance regression beyond that, to well below 9.1
performance.

Regards,
Dean

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