Major Description
The existing implementation of pivot translates into a single aggregation with one aggregate per distinct pivot value. When the number of distinct pivot values is large (say 1000+) this can get extremely slow since each input value gets evaluated on every aggregate even though it only affects the value of one of them.
I'm proposing an alternate strategy for when there are 10+ (somewhat arbitrary threshold) distinct pivot values. We do two phases of aggregation. In the first we group by the grouping columns plus the pivot column and perform the specified aggregations (one or sometimes more). In the second aggregation we group by the grouping columns and use the new (non public) PivotFirst aggregate that rearranges the outputs of the first aggregation into an array indexed by the pivot value. Finally we do a project to extract the array entries into the appropriate output column.
