Re: Statistics Import and Export

Gentle ping on this.

---
Hari Krishna Sunder

On Wed, May 14, 2025 at 1:30 PM Hari Krishna Sunder <hari(dot)db(dot)pg(at)gmail(dot)com>
wrote:

> 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
>>
>