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We can allow this even without any specific knowledge of the semantics of the window function, so long as pushed-down quals will either accept every row in a given window partition, or reject every such row. Because window functions act only within a partition, such a case can't result in changing the window functions' outputs for any surviving row. Eliminating entire partitions in this way obviously can reduce the cost of the window-function computations substantially. The fly in the ointment is that it's hard to be entirely sure whether this is true for an arbitrary qual condition. This patch allows pushdown if (a) the qual references only partitioning columns, and (b) the qual contains no volatile functions. We are at risk of incorrect results if the qual can produce different answers for values that the partitioning equality operator sees as equal. While it's not hard to invent cases for which that can happen, it seems to seldom be a problem in practice, since no one has complained about a similar assumption that we've had for many years with respect to DISTINCT. The potential performance gains seem to be worth the risk. David Rowley, reviewed by Vik Fearing; some credit is due also to Thomas Mayer who did considerable preliminary investigation.
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