We recently performed an upgrade from pg14 (14.18) to pg16 (16.9) and performed
the analyze-in-stages post upgrade. It has been noticed that some plans
changed to use hash joins instead of nested loops. Further investigation found
it was because the parent table of partitioned tables did not have stats.
After running an ANALYZE on the parent tables we got similar plan an execution
times as before.
I have two questions
1 - Why does analyze-in-stages not analyze the parent tables?
2 – What happens if we do not run analyze-in-stages post upgrade and just run
an analyze?
“It takes more time, and you don't have any statistics on a given table until
the ANALYZE on that table completes.
How long did "vacuumdb --analyze-only --jobs=$mumble your_db" take?”
Thanks – that makes sense. I understand what analyze in stages does just wish
it would include parent tables.
"vacuumdb --all --analyze-only --jobs=7" took about 75 minutes where the
analyze-in-stages after upgrade took 115 minutes. Neither of these activities
analyzed the parent tables.
Reading more and it seems vacuumdb doesn’t analyze parent tables and a manual
analyze on those is needed if we want better planner statistics.
