Zlatko Calusic <zlatko(at)iskon(dot)hr> writes:
> It is now NOT using the index, and I don't understand why? Queries are
> practically the same, tables are practically the same, why is postgres
> using indexes in the first case and not in the second?
Because it has substantially different ideas about the sizes of the
two tables --- notice the different estimated row counts. If you
haven't "vacuum analyzed" these tables recently, do so to bring the
planner's statistics up-to-date, and then see what you get. You may
also care to read the user's manual chapter about EXPLAIN,
http://www.postgresql.org/users-lounge/docs/7.0/postgres/c4888.htm
regards, tom lane