Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Jul 9, 2014 at 12:54 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> No, this is a completely different thing. The idea is that if the query
>> is grouping on outer-relation columns, you don't need to care if the inner
>> relation is unique or not, because it doesn't matter if there are multiple
>> matches.
> Oh... but that would only work if it were grouping without
> aggregation, right? Seems awfully narrow.
Right. Yeah, I was not that excited about its usefulness either.
However, the OP seems to think that it's a common optimization
(I wonder if he can cite chapter and verse on which other DBMSes
do it).
regards, tom lane