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Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions

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On Mon, 13 May 2024, Dimitrios Apostolou wrote:

On Sat, 11 May 2024, David Rowley wrote:

 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."

Thanks

Do I read that correctly, that I have to setup cron jobs to manually
analyze partitioned tables?


Dimitris







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