On 9 May 2016 at 18:46, David G. Johnston <david.g.johnston@xxxxxxxxx> wrote: > On Sunday, May 8, 2016, Sterpu Victor <victor@xxxxxxxx> wrote: >> >> Yes but it is very big. >> I don't understand why the select list is influencing the CPU usage. >> I was expecting that only the join and where clauses would influence CPU. >> > > PostgreSQL is smart enough to optimize away stuff that it knows doesn't > impact the final query result. To be more accurate with what David is saying, PostgreSQL will remove unused LEFT JOINed relations where the left joined relation can be proved to not duplicate rows from the right hand side. It would just be a matter of comparing the EXPLAINs from the query with all the SELECT items to the one with the single SELECT item to prove that this is what's happening. Please also note that this only occurs with LEFT JOINs It would also be quite helpful for people if you were to include a copy of the query. It's impossible to reverse engineer what that is from this EXPLAIN output. I see that your using a windowing function and performing a LIMIT 1, there may be ways to improve that just by selecting the single highest j1031101.validfrom row and performing the joins to the other table on that single row, but that will depend on which windowing function you're using as the function may require the other rows in the window frame to calculate the correct result. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
