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.
