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.
I think that improvement in cost calculation of sort should be a separate patch, not directly connected to this one. Postpone patches till other part will be ready to get max improvement for postponed ones doesn't seem to me very good, especially if it suggests some improvement right now.

Teodor Sigaev                      E-mail: teo...@sigaev.ru
                                      WWW: http://www.sigaev.ru/

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