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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Matthew Wakeling <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: plpgsql arrays
Date: Fri, 03 Apr 2009 10:48:31 -0400
Message-ID: <1138.1238770111@sss.pgh.pa.us> (raw)
In-Reply-To: <alpine.DEB.2.00.0904031526080.21772@aragorn.flymine.org>
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	<alpine.DEB.2.00.0904031526080.21772@aragorn.flymine.org>

Matthew Wakeling <matthew@flymine.org> writes:
> On Fri, 3 Apr 2009, Tom Lane wrote:
>> Not unless you have sorted the inputs in some way that has more 
>> knowledge than the "equal" operator represents.  Otherwise you can have 
>> elements drop out that might still be needed to match to a later 
>> left-hand element.

> Of course. You certainly have to choose a sort order that works. Sorting 
> by the start field would be sufficient in this case.

Uh, no, it wouldn't.  Visually:

	L1	-------------------------
	L2	-----------
	L3	---------------------

	R1	               --------

At L2, you'd conclude that you're done matching R1.

Intuitively, it seems like 1-D "overlaps" is a tractable enough
operator that you should be able to make something merge-like
work.  But it's more complicated than I think you realize.

			regards, tom lane



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