> From: Sailesh Krishnamurthy <[EMAIL PROTECTED]> > >>>>> "Tom" == Tom Lane <[EMAIL PROTECTED]> writes: > > Tom> The only real solution, of course, is to acquire cross-column > Tom> statistics, but I don't see that happening in the near > Tom> future. > > Another approach is a hybrid hashing scheme where we use a hash table > until we run out of memory at which time we start spilling to disk. In > other words, no longer use SortAgg at all .. > > Under what circumstances will a SortAgg consumer more IOs than a > hybrid hash strategy ?
Goetz Graefe did a heck of a lot of analysis of this, prior to his being snapped up by Microsoft. He also worked out a lot of the nitty-gritty for hybrid hash algorithms, extending the Grace hash for spill-to-disk, and adding a kind of recursion for really huge sets. The figures say that hybrid hash beats sort-aggregate, across the board. ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
