Re: Equivalent queries and the planner
От | Neil Conway |
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Тема | Re: Equivalent queries and the planner |
Дата | |
Msg-id | 1129426179.8219.44.camel@localhost.localdomain обсуждение исходный текст |
Ответ на | Equivalent queries and the planner ("John D. Burger" <john@mitre.org>) |
Список | pgsql-general |
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
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