Tom Lane <tgl@sss.pgh.pa.us> wrote:
> 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 ...
Question: when is the planner making the decision between in-memory and
on-disk, at planning-time or at execution time with the knowledge about
the real amount of tuples?
Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect. (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly." (unknown)
Kaufbach, Saxony, Germany, Europe. N 51.05082°, E 13.56889°