On Mon, Jun 3, 2013 at 5:03 AM, Craig Ringer <cr...@2ndquadrant.com> wrote: > I've seen cases on Stack Overflow and elsewhere in which disk merge > sorts perform vastly better than in-memory quicksort, so the user > benefited from greatly *lowering* work_mem.
I've heard of that happening on Oracle, when the external sort is capable of taking advantage of I/O parallelism, but I have a pretty hard time believing that it could happen with Postgres under any circumstances. Maybe if someone was extraordinarily unlucky and happened to hit quicksort's O(n ^ 2) worst case it could happen, but we take various measures that make that very unlikely. It might also have something to do with our "check for pre-sorted input" [1], but I'm still skeptical. [1] http://www.postgresql.org/message-id/caeylb_xn4-6f1ofsf2qduf24ddcvhbqidt7jppdl_rit1zb...@mail.gmail.com -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
