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Re: Review: listagg aggregate

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: "David E(dot)Wheeler" <david(at)kineticode(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Review: listagg aggregate
Date: 2010-01-24 18:43:54
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Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Fri, 2010-01-22 at 11:14 -0800, David E.Wheeler wrote:
>> No performance issues

> ISTM that this class of function is inherently dangerous performance
> wise. 

> * It looks incredibly easy to construct enormous lists. We should test
> the explosion limit of this to see how it is handled. Perhaps we need
> some parameter limits to control that, depending upon results.

> * Optimizer doesn't consider whether the result type of an aggregate get
> bigger as the aggregate processes more rows. If we're adding this
> function we should give some thought in that area also, or at least a
> comment to note that it can and will cause the optimizer problems in
> complex queries.

We have that problem already with array_agg(), and I don't recall many
complaints about it.  It might be worth worrying about at some point,
but I don't think it's reasonable to insist that it be fixed before
any more such aggregates are created.

I agree that testing-to-failure would be a good idea just to be sure it
fails cleanly.

			regards, tom lane

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