On Fri, Apr 3, 2009 at 11:52 AM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: > On Fri, Apr 3, 2009 at 9:17 AM, Matthew Wakeling <matthew@xxxxxxxxxxx> wrote: >> >> So, I have a view. The query that the view uses can be written two different >> ways, to use two different indexes. Then I use the view in another query, >> under some circumstances the first way will be quick, and under other >> circumstances the second way will be quick. >> >> What I want to know is, can I create a view that has both queries, and >> allows the planner to choose which one to use? The documentation seems to >> say so in http://www.postgresql.org/docs/8.3/interactive/querytree.html (the >> rule system "creates zero or more query trees as result"), but doesn't say >> how one would do it. > > yes. > > create view v as > select * from > ( > select true as b, pg_sleep(1)::text > union all > select false as b, pg_sleep(1)::text > ) q; > > recent versions of pg are smart enough to optimize (in some cases): > select * from v where b; oop, I read your question wrong. for the above to work, _you_ have to choose the plan, not the planner. I think it still might be possible so long as you can deterministically figure out (say, as the result of a function) which query you want the planner to choose using a form of the above technique. merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
