From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Віталій Тимчишин <tivv00(at)gmail(dot)com> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Planner not using column limit specified for one column for another column equal to first |
Date: | 2010-04-16 14:19:47 |
Message-ID: | 8534.1271427587@sss.pgh.pa.us |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-performance |
=?KOI8-U?B?96bUwcymyiD0yc3eydvJzg==?= <tivv00(at)gmail(dot)com> writes:
> I've thought and someone in this list've told me that this should be done
> automatically.
No, that's not true. We do make deductions about transitive equalities,
ie, given WHERE a=b AND b=c the planner will infer a=c and use that if
it's helpful. We don't make deductions about inequalities such as a>c.
In theory there's enough information available to do so, but overall
trying to do that would probably waste more cycles than it would save.
You'd need a lot of expensive new planner infrastructure, and in the
vast majority of queries it wouldn't produce anything very helpful.
As was pointed out, even if we had such logic it wouldn't apply in this
example, because the equality conditions aren't real equalities but
OUTER JOIN conditions.
regards, tom lane
From | Date | Subject | |
---|---|---|---|
Next Message | Josh Kupershmidt | 2010-04-16 14:39:07 | Re: stats collector suddenly causing lots of IO |
Previous Message | Віталій Тимчишин | 2010-04-16 13:58:22 | Re: Planner not using column limit specified for one column for another column equal to first |