Robert Haas <robertmh...@gmail.com> writes: > On Thu, Feb 11, 2010 at 7:48 AM, Bart Samwel <b...@samwel.tk> wrote: >> Because that's the >> underlying assumption of the "ratio" criterion -- that re-planning with >> filled-in parameters takes about as much time as the initial planning run >> took.
> We only want to replan when replanning is relatively cheap compared to > execution, Well, no, consider the situation where planning takes 50 ms, the generic plan costs 100ms to execute, but a parameter-specific plan would take 1ms to execute. Planning is very expensive compared to execution but it's still a win to do it. The problem that we face is that we don't have any very good way to tell whether a fresh planning attempt is likely to yield a plan significantly better than the generic plan. I can think of some heuristics --- for example if the query contains LIKE with a parameterized pattern or a partitioned table --- but that doesn't seem like a particularly nice road to travel. A possible scheme is to try it and keep track of whether we ever actually do get a better plan. If, after N attempts, none of the custom plans were ever more than X% cheaper than the generic one, then give up and stop attempting to produce custom plans. Tuning the variables might be challenging though. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers