On Fri, 2005-14-10 at 09:43 -0400, John D. Burger wrote: > I believe these queries are exactly equivalent, but I presume the > planner doesn't know that. > explain select gazPlaceID from gazPlaces > where gazPlaceID not in (select gazPlaceID from gazContainers); > explain select gazPlaceID from gazPlaces > except select gazPlaceID from gazContainers; Yeah, query optimization for set operations is currently quite primitive; the above transformation is not yet implemented. > In general, there are lots of ways to express the same abstract > information need in SQL, and I assumed that there were some set of > (probably incomplete) equivalencies encoded somewhere. Is this so? I don't know of a canonical list of planner transformations. There are some presentations on planner internals that touch on this, which is better than nothing: http://neilc.treehou.se/optimizer.pdf http://conferences.oreillynet.com/presentations/os2003/lane_tom.pdf -Neil ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
