> The hstore type is probably the closest you'd get to an associate array in PostgreSQL

No, a table (possibly a temp table with the results of a particular query) with an appropriately defined primary key (or other unique index) would be the closest you can get to an associative array -- since a relation with n candidate keys is exactly identical to n different associative arrays with the same data but different key/value splits.

But an array of a composite type drawn in such a way that some set of columns is guaranteed unique is pretty much the same thing from a data perspective, even though you may need to do work on the client side to load it into a structure that supports associative array operations (e.g., efficient key/value lookups).