On Fri, Mar 23, 2018 at 12:41:35PM +0100, Laurenz Albe wrote: ! https://www.postgresql.org/docs/current/static/explicit-joins.html ! states towards the end of the page that the search tree grows ! exponentially with the number of relations, and from_collapse_limit ! can be set to control that. Yes, I read that page. ! > In my case, planning uses 1 or 2% of the cycles needed for ! > execution; that seems alright to me. ! > And, as said above, I cannot see why my queries might be an ! > atypical case (I don't think they are). ! > ! > If somebody would like to get a hands-on look onto the actual ! > case, I'd be happy to put it online. ! ! It seems like you are barking up the wrong tree. ! ! Your query does not take long because of the many relations in the ! FROM list, but because the optimizer makes a wrong choice. Exactly! And I am working hard in order to understand WHY this happens. ! The correct solution is *not* to set from_collapse_limit = 1, but ! to find and fix the problem that causes the optimizer to make a ! wrong choice. ! ! If you send the query and the output of ! EXPLAIN (ANALYZE, BUFFERS) SELECT ... ! we have a chance of telling you what's wrong. Your viewpoint would be preferrable, only I am lacking any idea on where there could be such a problem that would make up a root cause. I will gladly follow Your suggestion; data is underway. P.
