Gentle ping on this. --- Hari Krishna Sunder
On Wed, May 14, 2025 at 1:30 PM Hari Krishna Sunder <hari.db...@gmail.com> wrote: > Thanks Nathan. > Here is the patch with a comment. > > On Wed, May 14, 2025 at 8:53 AM Nathan Bossart <nathandboss...@gmail.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 >> >
