"Benjamin Krajmalnik" <kraj@servoyant.com> writes:
> A little background - I have various multi-column indexes whenever I
> have queries which restrict the output based on the values of the 2
> fields (for example, a client code and the date of a transaction).
> Is there a performance gain using this approach as opposed to using 2
> separate indexes, one on the first column and one on the second column?
Maybe, maybe not ... it's going to depend on a bunch of factors, one of
which is what your update load is like compared to the queries that read
the indexes. There's a bit of coverage of this in the fine manual: see
http://www.postgresql.org/docs/8.4/static/indexes-multicolumn.html
and the next few pages.
The short of it is that there's no substitute for doing your own
experiments for your own application ...
regards, tom lane