On Thu, Jul 30, 2015 at 4:26 AM, Greg Stark <st...@mit.edu> wrote:
> I'm a bit confused where the big win comes from though. Is what's going on
> that the external sort only exceeded memory by a small amount  so nearly all
> the tuples are still in memory?

Yes, that's why this can be much faster just as the work_mem threshold
is crossed. You get an "almost internal" sort, which means you can
mostly quicksort, and you can avoid dumping most tuples. It's still a
pretty nice win when less than half of tuples fit in memory, though --
just not as nice. Below that, the optimization isn't used.

Peter Geoghegan

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