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Re: Window-functions patch handling of aggregates

From: "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>
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 19:29:21
Message-ID: 1230319761.6674.2.camel@jd-laptop.pragmaticzealot.org (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, 2008-12-26 at 14:17 -0500, Tom Lane wrote:
> Greg Stark <greg(dot)stark(at)enterprisedb(dot)com> writes:
> > Yeah, it seems like adding a flag like iswindowable to aggregate  
> > functions is the safest option.
> 

> 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?
> 
> 			regards, tom lane
> 

I believe the goal should be correctness but why not both? Fix what we
can and put in place a "work around" that would be removed in 8.5?

Joshua D. Drake


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