| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Moshe Jacobson <moshe(at)neadwerx(dot)com>, Postgres-Bugs <pgsql-bugs(at)postgresql(dot)org>, Robert Mowlavi <robert(at)neadwerx(dot)com>, Chris Autry <chris(at)neadwerx(dot)com> |
| Subject: | Re: LEFT JOINs not optimized away when not needed |
| Date: | 2014-07-09 17:33:02 |
| Message-ID: | 18920.1404927182@sss.pgh.pa.us |
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| Lists: | pgsql-bugs pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Jul 9, 2014 at 12:54 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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
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