Alvaro Herrera <alvhe...@commandprompt.com> writes: > Tom Lane escribió: >> Yeah, it could definitely run slower than the existing code --- in >> particular the combination of all three (FOR UPDATE ORDER BY LIMIT) >> would tend to become a seqscan-and-sort rather than possibly just >> reading one end of an index. However, I quote the old aphorism that >> it can be made indefinitely fast if it doesn't have to give the right >> answer. The reason the current behavior is fast is it's giving the >> wrong answer :-(
> So this probably merits a warning in the release notes for people to > check that their queries continue to run with the performance they > expect. One problem with this is that there isn't any good way for someone to get back the old behavior if they want to. Which might be a perfectly reasonable thing, eg if they know that no concurrent update is supposed to change the sort-key column. The obvious thing would be to allow select * from (select * from foo order by col limit 10) ss for update; to apply the FOR UPDATE last. Unfortunately, that's not how it works now because the FOR UPDATE will get pushed down into the subquery. I was shot down when I proposed a related change, a couple weeks ago http://archives.postgresql.org/message-id/7741.1255278...@sss.pgh.pa.us but it seems like we might want to reconsider. 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
