8b49a6044d
The initial commit of ordered-set aggregates just did all the setup work afresh each time the aggregate function is started up. But in a GROUP BY query, the catalog lookups need not be repeated for each group, since the column datatypes and sort information won't change. When there are many small groups, this makes for a useful, though not huge, performance improvement. Per suggestion from Andrew Gierth. Profiling of these cases suggests that it might be profitable to avoid duplicate lookups within tuplesort startup as well; but changing the tuplesort APIs would have much broader impact, so I left that for another day.
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