If you have only recently analyzed the tables in the query, close your psql session (if that's what you were using) and then restart it. I've gotten burned by asking a query using the function, which I believe is when PG creates the plan for the function, and then making significant changes to the tables behind it (new index, bulk insert, etc.). By starting a new session, the function will be re-planned according to up to date statistics or using newly created indices.
