I have no idea why this happening. But I suspect the parallel requires more internal machine resources like CPU etc because you can faster retrieve the disk data from the one hand but you ought to spend more resources for maintaining several threads and theirs coordination (px coordinator process in Oracle terms) from another one. Thus there could be more serious hardware requirements even just to keep performance the same. I believe that the real benefit of the parallel will be shown when you have pair of large and wide tables (30M or more each) with hash join (typical task for mart construction) but such class of databases is supposed to be big and required enough resources initially.
From: Mariel Cherkassky <mariel.cherkassky@gmail.com> Sent: Monday, December 16, 2019 2:48 PM To: Jeff Janes <jeff.janes@gmail.com> Cc: Andrew Zakharov <Andrew898@mail.ru>; pgsql-performance@lists.postgresql.org Subject: Re: performance degredation after upgrade from 9.6 to 12
Hey Jeff,Andrew,
I continued testing the 12version vs the 96 version and it seems that there is almost non diff and in some cases pg96 is faster than 12. I compared the content of pg_stat_statements after each test that I have done and it seems that the db time is almost the same and sometimes 96 is faster by 5%.
Any idea why there isnt any improvement even when I enabled the parallel params in 12 ?
