| From: | Hari Krishna Sunder <hari(dot)db(dot)pg(at)gmail(dot)com> |
|---|---|
| To: | Nathan Bossart <nathandbossart(at)gmail(dot)com> |
| Cc: | Corey Huinker <corey(dot)huinker(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Robert Treat <rob(at)xzilla(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(at)paquier(dot)xyz>, jian he <jian(dot)universality(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Stephen Frost <sfrost(at)snowman(dot)net>, Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>, Peter Smith <smithpb2250(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, alvherre(at)alvh(dot)no-ip(dot)org |
| Subject: | Re: Statistics Import and Export |
| Date: | 2025-05-14 00:01:02 |
| Message-ID: | CAAeiqZ0o2p4SX5_xPcuAbbsmXjg6MJLNuPYSLUjC=Wh-VeW64A@mail.gmail.com |
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| Lists: | pgsql-hackers |
We found a minor issue when testing statistics import with upgrading from
versions older than v14. (We have VACUUM and ANALYZE disabled)
3d351d916b20534f973eda760cde17d96545d4c4
<https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=3d351d916b20534f973eda760cde17d96545d4c4>
changed
the default value for reltuples from 0 to -1. So when such tables are
imported they get the pg13 default of 0 which in pg18 is treated
as "vacuumed and seen to be empty" instead of "never yet vacuumed". The
planner then proceeds to pick seq scans even if there are indexes for these
tables.
This is a very narrow edge case and the next VACUUM or ANALYZE will fix it
but the perf of these tables immediately after the upgrade is considerably
affected.
Can we instead use -1 if the version is older than 14, and reltuples is 0?
This will have the unintended consequence of treating a truly empty table
as "never yet vacuumed", but that should be fine as empty tables are going
to be fast regardless of the plan picked.
PS: This is my first patch, so apologies for any issues with the patch.
On Fri, Apr 4, 2025 at 7:06 PM Nathan Bossart <nathandbossart(at)gmail(dot)com>
wrote:
> On Fri, Apr 04, 2025 at 07:32:48PM -0400, Corey Huinker wrote:
> > This patch shrinks the array size to 1 for versions < 9.4, which keeps
> the
> > modern code fairly elegant.
>
> Committed.
>
> --
> nathan
>
>
>
| Attachment | Content-Type | Size |
|---|---|---|
| 0001-Stats-import-Fix-default-reltuples-on-versions-older.patch | application/octet-stream | 1.2 KB |
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