Re: doc: explain pgstatindex fragmentation

From: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
To: Benoit Lobréau <benoit(dot)lobreau(at)dalibo(dot)com>, Frédéric Yhuel <frederic(dot)yhuel(at)dalibo(dot)com>, Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: doc: explain pgstatindex fragmentation
Date: 2025-01-27 09:21:39
Message-ID: 6edfcc829d0c11be54569cd0f073ef8c02d2d2aa.camel@cybertec.at
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On Mon, 2025-01-27 at 10:13 +0100, Benoit Lobréau wrote:
> On 1/25/25 7:07 PM, Laurenz Albe wrote:
> > Looks good to me.  I have one question left: the explanation for the performance
> > penalty of a high leaf fragmentation sounds like it would only be relevant for
> > disks where sequential reads are faster.  If that is correct, perhaps it would be
> > worth mentioning.
>
> Frederic noticed a performance hit even for on his laptop with a SSD.
>
> On Fri, 2025-01-24 at 15:41 +0100, Frédéric Yhuel wrote:
>  > I've noticed that maximum leaf_fragmentation can have a huge impact on
>  > a range index-only scan, when reading all blocs from disks, even on my
>  > laptop machine with SSD, but I don't know if this is the right place
>  > to document this?
>
> He reported to our team, that he did a test with two indexes on the same
> data. They had the same density but one had no fragmentation while the
> other had 100%. He got an execution time of ~90ms (0 frag) vs ~340ms
> 100% frag).
>
> I get similar result with my laptor (except my disk is significantly
> worse: ~152ms vs ~833ms).

Thanks for checking.
I'll set the patch "ready for committer".
I personally would still like to know how fragmentation slows down performance.

Yours,
Laurenz Albe

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