I wrote: > I instrumented the code in setrefs.c, and found that during the > core regression tests this patch estimates correctly in 2103 > places while guessing wrongly in 54, so that seems like a pretty > good step forward.
On second thought, that's not a terribly helpful summary. Breaking things down to the next level, there were 1088 places where we correctly guessed a subquery isn't trivial (so no change from current behavior, which is correct) 1015 places where we correctly guessed a subquery is trivial (hence, improving the cost estimate from before) 40 places where we incorrectly guessed a subquery isn't trivial (so no change from current behavior, although that's wrong) 14 places where we incorrectly guessed a subquery is trivial (hence, incorrectly charging zero for the SubqueryScan) 1015 improvements to 14 disimprovements isn't a bad score. I'm a bit surprised there are that many removable SubqueryScans TBH; maybe that's an artifact of all the "SELECT *" queries. regards, tom lane