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

Andreas Kretschmer <akretschmer(at)spamfence(dot)net> writes:
> 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,

> 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?

The planner doesn't make that decision. tuplesort.c always starts in
in-memory mode, and flips to on-disk when the actual amount of data in
its care exceeds work_mem. The planner guesses whether that will happen
while making cost estimates, but it's only an estimate.

regards, tom lane