On Tue, Nov 12, 2019 at 12:20:10PM -0700, Michael Lewis wrote:
> It is very interesting to me that the optimizer chose a parallel sequential
> scan rather than an index scan on either of your indexes that start
> with project_id that also reference trashed_at.
Maybe because of low correlation on any of those columns?
https://wiki.postgresql.org/wiki/Slow_Query_Questions#Statistics:_n_distinct.2C_MCV.2C_histogram
SELECT (SELECT sum(x) FROM unnest(most_common_freqs) x) frac_MCV, tablename, attname, inherited, null_frac, n_distinct,
array_length(most_common_vals,1)n_mcv, array_length(histogram_bounds,1) n_hist, correlation FROM pg_stats WHERE
tablename='subscriptions'AND attname IN ('project_id','tags') ORDER BY 1 DESC;
Maybe clustering the table on project_id (and ANALYZEing) would help, but that
might need to be done consistently.
Michael previously suggested partitioning which, if done on project_id,
would then no longer need to be specially CLUSTERed.
Is the plan for the fast query the same as in August ?
https://www.postgresql.org/message-id/CAFvCgN4UijKTYiOF61Tyd%2BgHvF_oqnMabatS9%2BDcX%2B_PK2SHRw%40mail.gmail.com
Justin