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Re: [planner] Ignore "order by" in subselect if parrent do count(*)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Craig James <cjames(at)emolecules(dot)com>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, "Marcin Miros*aw" <marcin(at)mejor(dot)pl>, pgsql-performance(at)postgresql(dot)org
Subject: Re: [planner] Ignore "order by" in subselect if parrent do count(*)
Date: 2012-03-01 21:29:24
Message-ID: 17997.1330637364@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Craig James <cjames(at)emolecules(dot)com> writes:
> On Thu, Mar 1, 2012 at 9:50 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Considering that ORDER BY in a subquery isn't even legal per spec,

> That's surprising ... normally it won't affect the result, but with an
> offset or limit it would.  Does the offset or limit change the "not
> even legal" part?

Well, actually, the SQL standard didn't have anything comparable to
offset/limit until SQL:2008, either.  But I have to take back my
statement above.  It wasn't legal in SQL99, but evidently they added it
in the 2003 or 2008 edition, presumably to go with the limit
functionality.

Anyway, the long and the short of it is that people depend on ORDER BY
in subqueries to be honored, and we're not going to break that.

			regards, tom lane

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