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Re: Window-functions patch handling of aggregates

From: David Fetter <david(at)fetter(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Greg Stark <greg(dot)stark(at)enterprisedb(dot)com>,Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Window-functions patch handling of aggregates
Date: 2008-12-26 20:24:24
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Lists: pgsql-hackers
On Fri, Dec 26, 2008 at 02:17:29PM -0500, Tom Lane wrote:

> So the alternatives I see are:
> 1. Go back to Hitoshi's plan of passing WindowAggState to the
> aggregates.  This will require changing every one of the ten aggregates
> in the core distro, as well as every third-party aggregate that has
> a similar optimization; and we just have to keep our fingers crossed
> that anyone who's taking a short-cut will fix their code before it
> fails in the field.
> 2. Use an intermediate dummy AggState as I have in my version, but
> document some convention for telling this from a "real" AggState
> when needed.  (Not hard, we just pick some field that would never be
> zero in a real AggState and document testing that.)  This is certainly
> on the ugly side, but it would very substantially cut the number of
> places that need changes.  Only aggregates that are doing something
> irreversible in their final-functions would need to be touched.
> If we were working in a green field then #1 would clearly be the
> preferable choice, but worrying about compatibility with existing
> third-party aggregates is making me lean to #2.  Comments?

Exactly how large is this third-party aggregate problem?  Rather than
support a huge wart, we could just tell people starting now and
prominently in the release notes that such things need a re-do and
point to examples of how it's done.

The case this won't work for is where a vendor of a proprietary C
extension is gone and/or won't update their stuff for 8.4, and it
seems to me that we can't, and shouldn't try to, take responsibility
for that use case anyhow.

David Fetter <david(at)fetter(dot)org>
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david(dot)fetter(at)gmail(dot)com

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