On Thu, May 21, 2009 at 12:21 AM, Josh Berkus <j...@agliodbs.com> wrote: >> It appears that this statement has been in our documentation since Tom >> Lane added FROM_COLLAPSE_LIMIT (back then, it was capitalized) on >> January 25, 2003 (9bf97ff426de9), but I can't find any justification >> for it anywhere. I think we either need to justify this advice, or >> remove it. > > ... trying to remember why I wrote that ... what would happen if > FROM_COLLAPSE_LIMIT was *more* than GEQO_THRESHOLD?
The two variables do different things, so there's nothing particularly magical about which one is larger AFAICS. I believe that if you make from_collapse_limit larger than geqo_threshold, then GEQO might be asked to plan a query into which subqueries have been pulled up. But that's not obviously bad; the alternative is planning the subquery separately and first, which at least for the very small number of cases that I've tested seems to be quite a bit worse. Apparently before from_collapse_limit was added the behavior existed, but the thereshold was geqo_threshold/2. So someone had a reason for believing that when the join nest got too large, not pulling up subqueries was a superior coping strategy versus invoking GEQO. I just don't know what the reason is, or whether it's still valid. ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
