On Sat, 11 May 2024 at 13:11, Dimitrios Apostolou <jimis@xxxxxxx> wrote: > Indeed that's an awful estimate, the table has more than 1M of unique > values in that column. Looking into pg_stat_user_tables, I can't see the > partitions having been vacuum'd or analyzed at all. I think they should > have been auto-analyzed, since they get a ton of INSERTs > (no deletes/updates though) and I have the default autovacuum settings. > Could it be that autovacuum starts, but never > finishes? I can't find something in the logs. It's not the partitions getting analyzed you need to worry about for an ndistinct estimate on the partitioned table. It's auto-analyze or ANALYZE on the partitioned table itself that you should care about. If you look at [1], it says "Tuples changed in partitions and inheritance children do not trigger analyze on the parent table." > In any case, even after the planner decides to execute the terrible plan > with the parallel seqscans, why doesn't it finish right when it finds 10 > distinct values? It will. It's just that Sorting requires fetching everything from its subnode. David [1] https://www.postgresql.org/docs/16/routine-vacuuming.html#VACUUM-FOR-STATISTICS
