On Jun 3, 2013, at 6:45 PM, Craig Ringer <cr...@2ndquadrant.com> wrote: > On 06/04/2013 05:27 AM, Peter Geoghegan wrote: >> 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. > IIRC it's usually occurred with very expensive comparison operations. > > I'll see if I can find one of the SO cases.
FWIW, I've definitely seen this behavior in the past, on really old versions (certainly pre-9, possibly pre-8). IIRC there's some kind of compression or something used with on-disk sorts. If that's correct then I think what's happening is that the "on-disk" sort that fits into cache is actually using less memory than quicksort. Or perhaps it was just a matter of memory locality within each tape. It's been too long since I looked at it. :( -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
