Re: Statistics Import and Export

Thanks Nathan.
Here is the patch with a comment.

On Wed, May 14, 2025 at 8:53 AM Nathan Bossart <nathandbossart(at)gmail(dot)com>
wrote:

> On Tue, May 13, 2025 at 05:01:02PM -0700, Hari Krishna Sunder wrote:
> > 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.
>
> There was a similar report for vacuumdb's new --missing-stats-only option.
> We fixed that in commit 9879105 by removing the check for reltuples != 0,
> which means that --missing-stats-only will process empty tables.
>
> > 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.
>
> I'm inclined to agree that we should do this. Even if it's much more
> likely that 0 means empty versus not-yet-processed, the one-time cost of
> processing some empty tables doesn't sound too bad. In any case, since
> this only applies to upgrades from <v14, that trade-off should dissipate
> over time.
>
> > PS: This is my first patch, so apologies for any issues with the patch.
>
> It needs a comment, but otherwise it looks generally reasonable to me after
> a quick glance.
>
> --
> nathan
>