Re: [HACKERS] [PATCH] Incremental sort

On 2018-04-07 12:06:52 -0400, Tom Lane wrote:
> Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> writes:
> > On Wed, Mar 28, 2018 at 6:38 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
> > wrote:
> >> Can we have a recap on what the patch *does*?
>
> > Ggeneral idea hasn't been changed much since first email.
> > Incremental sort gives benefit when you need to sort your dataset
> > by some list of columns while you alredy have input presorted
> > by some prefix of that list of columns. Then you don't do full sort
> > of dataset, but rather sort groups where values of prefix columns
> > are equal (see header comment in nodeIncremenalSort.c).
>
> I dunno, how would you estimate whether this is actually a win or not?
> I don't think our model of sort costs is anywhere near refined enough
> or accurate enough to reliably predict whether this is better than
> just doing it in one step. Even if the cost model is good, it's not
> going to be better than our statistics about the number/size of the
> groups in the first column(s), and that's a notoriously unreliable stat.
>
> Given that we already have more than enough dubious patches that have
> been shoved in in the last few days, I'd rather not pile on stuff that
> there's any question about.

I don't disagree with any of that. Just wanted to pipe up to say that
there's a fair argument to be made that this patch, which has lingered
for years, "deserves" more to mature in tree than some of the rest.

Greetings,

Andres Freund