Alvaro Herrera <alvherre@xxxxxxxxxxxxxxxxx> writes: > Are you saying that an indexscan "Filter" only acts after getting the > heap tuple? Correct. > If that's the case, then there's room for optimization > here, namely if the affected column is part of the index key, then we > could do the filtering before fetching the heap tuple. Only if the index is capable of disgorging the original value of the indexed column, a fact not in evidence in general (counterexample: polygons indexed by their bounding boxes in an r-tree). But yeah, it's interesting to think about applying filters at the index fetch step for index types that can hand back full values. This has been discussed before --- I think we had gotten as far as speculating about doing joins with just index values. See eg here: http://archives.postgresql.org/pgsql-hackers/2004-05/msg00944.php A lot of the low-level concerns have already been dealt with in order to support bitmap indexscans, but applying non-indexable conditions before fetching from the heap is still not done. regards, tom lane
