Re: work_mem = 900MB but Sort Method: external merge Disk: 304008kB

Reid Thompson <Reid.Thompson@ateb.com> writes:
> What am I missing that causes this to resort to sorting on disk?

The in-memory space required to sort N tuples can be significantly
larger than the on-disk space, because the latter representation is
optimized to be small and the in-memory representation not so much.
I haven't seen a 3X differential before, but it's not outside the realm
of reason, especially for narrow rows like these where it's all about
the overhead.  I suspect if you crank work_mem up still more, you'll see
it switch over.  It flips to on-disk sort when the in-memory
representation exceeds the limit ...

            regards, tom lane