FAQ

On Jun 3, 2013, at 6:45 PM, Craig Ringer wrote:
On 06/04/2013 05:27 AM, Peter Geoghegan wrote:
On Mon, Jun 3, 2013 at 5:03 AM, Craig Ringer 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. :(

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