On Mon, Jun 3, 2013 at 5:03 AM, Craig Ringer <craig@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_RiT1zBJ6A@mail.gmail.com
--
Peter Geoghegan