Re: doc: explain pgstatindex fragmentation

Laurenz Albe <laurenz.albe@cybertec.at>

From: Laurenz Albe <laurenz.albe@cybertec.at>

To: Benoit Lobréau <benoit.lobreau@dalibo.com>, Frédéric Yhuel <frederic.yhuel@dalibo.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-01-27T09:21:39Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

doc: explain pgstatindex fragmentation
- 960135114629 19 (unreleased) landed

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