On Mon, Sep 11, 2017 at 8:32 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Sun, Sep 10, 2017 at 9:39 PM, Peter Geoghegan <p...@bowt.ie> wrote: >> To be clear, you'll still need to set replacement_sort_tuples high >> when testing RS, to make sure that we really use it for at least the >> first run when we're expected to. (There is no easy way to have >> testing mechanically verify that we really do only have one run in the >> end with RS, but I assume that such paranoia is unneeded.) > > I seem to recall that raising replacement_sort_tuples makes > replacement selection look worse in some cases -- the optimization is > more likely to apply, sure, but the heap is also bigger, which hurts.
But that was only because work_mem was set relatively high. If you're going to test work_mem values that are slightly on the high side for replacement selection (like, 8MB or 32MB), then increasing replacement_sort_tuples ensures that replacement selection is actually used for at least the first run, and you don't waste time comparing a behavior (quicksorting runs) to itself. All that matters is whether or not replacement_sort_tuples exceeds the number of tuples that the first run would have if quicksorted immediately. replacement_sort_tuples is only ever used to answer a single yes/no question. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers