"Brandon Aiken" <BAiken@xxxxxxxxxxxxxxx> writes: > If you have, say, an index(x, y) then that index will often double as an > index(x). It will generally not double as an index(y). It's not hard to understand why, if you think about the sort ordering of a double-column index: x y 1 1 1 2 1 3 2 1 2 2 2 3 3 1 ... All similar values of x are brought together, so scanning the index for x alone works just the same as it would in a one-column index ... the index entries are bigger so it's marginally less efficient, but only marginally. On the other hand, the entries for a specific value or range of y will be scattered all over the index, so it's almost useless to use the index for a search on y alone. As of PG 8.1 or 8.2 (I forget) the optimizer will *consider* using such an index for a y-only query, but it'll nearly always decide it's a bad idea. regards, tom lane
