Hi Eddy
Perhaps a slightly naive suggestion .... have you considered
converting the query to a small stored procedure ('function' in
Postgres speak)? You can pull the location values, and then iterate
over a query like this:
select userid from users where location=:x
which is more-or-less guaranteed to use the index.
I had a somewhat similar situation recently, where I was passing in a
list of id's (from outwith Postgres) and it would on occasion avoid
the index in favour of a full table scan .... I changed this to
iterate over the id's with separate queries (in Java, but using a
function will achieve the same thing) and went from one 5 minute query
doing full table scan to a handful of queries doing sub-millisecond
direct index lookups.
Cheers
Dave