index usage in joins q'n
http://www.postgresql.org/docs/8.2/interactive/indexes-intro.html
states that "Indexes can moreover be used in join searches. Thus, an
index defined on a column that is part of a join condition can
significantly speed up queries with joins."
Does this mean that a condition like "WHERE ... [AND] lhs.a=rhs.b [AND]
..." where rhs.b is already unique-indexed, also requires (non-unique)
index on lhs.a for maximal join speed? Otherwise why would they want to
say that?
Thanks.
On Sat, Nov 03, 2007 at 11:42:39AM +0400, rihad wrote:
Does this mean that a condition like "WHERE ... [AND] lhs.a=rhs.b [AND]
..." where rhs.b is already unique-indexed, also requires (non-unique)
index on lhs.a for maximal join speed? Otherwise why would they want to
say that?
No, as long as one of the two columns is indexed it can help. An index
on both might help if you need a lot of rows but it's a bit hard to
construct a situation where it'd be obvious.
Have a nice day,
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
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