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 >
0001-Stats-import-Fix-default-reltuples-on-versions-older.patch
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