2010/2/11 Tom Lane <t...@sss.pgh.pa.us>: > 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.
I afraid so every heuristic is bad. Problem is identification of bad generic plan. And nobody ensure, so non generic plan will be better than generic. Still I thing we need some way for lazy prepared statements - plan is generated everytime with known parameters. Other idea: some special debug/test mod, where pg store generic plan for every prepared statement, and still generate specific plan. When the prices are different, then pg produces a warning. This can be slower, but can identify problematic queries. It could be implemented as contrib module - some like autoexplain. regards Pavel > > 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
