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

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Reid Thompson <Reid(dot)Thompson(at)ateb(dot)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
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