> > 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 is spelled out in the docs: > > https://urldefense.com/v3/__https://www.postgresql.org/docs/current/pgupgrade.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvnFFR6Ws$<https://urldefense.com/v3/__https:/www.postgresql.org/docs/current/pgupgrade.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvnFFR6Ws$> > > Emphasis added > > "Using vacuumdb --all --analyze-only can efficiently generate such > statistics, and the use of --jobs can speed it up. Option > --analyze-in-stages can be used to generate **minimal statistics** > quickly. If vacuum_cost_delay is set to a non-zero value, this can be > overridden to speed up statistics generation using PGOPTIONS, e.g., > PGOPTIONS='-c vacuum_cost_delay=0' vacuumdb ...." > > and from here: > > https://urldefense.com/v3/__https://www.postgresql.org/docs/current/app-vacuumdb.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvvC7gfd0$<https://urldefense.com/v3/__https:/www.postgresql.org/docs/current/app-vacuumdb.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvvC7gfd0$> > > "--analyze-in-stages > > Only calculate statistics for use by the optimizer (no vacuum), > like --analyze-only. Run three stages of analyze; the first stage uses > the lowest possible statistics target (see default_statistics_target) to > produce usable statistics faster, and subsequent stages build the full > statistics. > > This option is only useful to analyze a database that currently has > no statistics or has wholly incorrect ones, such as if it is newly > populated from a restored dump or by pg_upgrade. Be aware that running > with this option in a database with existing statistics may cause the > query optimizer choices to become transiently worse due to the low > statistics targets of the early stages. Well, that wouldn't explain why it doesn't work on partitioned tables. I am under the impression that it should. Derek, can cou share the pg_stats entries for the partitioned table? Yours, Laurenz Albe There are no entries in pg_stats for the parent table until after I manually run an analyze on it – Example below => select relname, reltuples, relkind from pg_class where relname ~ '^chapter_[0-9]+$' or relname='chapter' order by 1; relname | reltuples | relkind -------------+-----------+--------- chapter | -1 | p chapter_1 | 4 | r chapter_10 | 4 | r chapter_100 | 30 | r chapter_101 | 15 | r chapter_102 | 15 | r … => select count(*) from pg_stats where tablename='chapter'; count ------- 0 (1 row) => analyze chapter; ANALYZE => select relname, reltuples, relkind from pg_class where relkind ='p' and relname='chapter'; relname | reltuples | relkind ---------+-----------+--------- chapter | 7589 | p (1 row) => select count(*) from pg_stats where tablename='chapter'; count ------- 49 (1 row) toy_epc_stg_1_db=>
