Re: Whether to back-patch fix for aggregate transtype width estimates

On Fri, Jun 17, 2016 at 10:41 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> While fixing the recent unpleasantness over parallel polymorphic
> aggregates, I discovered that count_agg_clauses_walker's consideration
> of an aggregate argument's typmod in estimating transition space
> consumption has been broken since commit 34d26872e (which changed
> Aggref.args from a simple expression list to a list of TargetEntry).
> It had been looking at "exprTypmod((Node *) linitial(aggref->args))",
> and nobody taught it that there was now a TargetEntry in the way that
> it needed to look through to get a correct answer.
>
> Ordinarily I'd just summarily back-patch a fix, but that commit shipped
> in 9.0, which means it's been broken a long time. I'm worried that
> back-patching a change might be more likely to destabilize plan choices
> than to do anything anybody's happy about.
>
> Thoughts?

I suspect the consequences here aren't too bad, or someone would have
noticed by now. So I would be tempted to leave it alone in
back-branches. But I might change my mind if it's actually awful...

--
Robert Haas
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