Andreas Kretschmer <[email protected]> writes:
> Tom Lane <[email protected]> wrote:
>> Reid Thompson <[email protected]> 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
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