Forcing postgres NOT to use sequential scan, trough JDBC

How do I tell postgres not to use sequential scan, trough jdbc?

I tried to include 'SET enable_seqscan TO false;' at the begining of the
plpgsql function, but that seems to work only if I call the function
trough the psql, and only for the subsequent calls in the current
session. I 'know' that postgres is using sequential scan because
function call takes cca 400ms. When I force postgres not to use
sequential scan, the function takes around 5-10ms.

I even tried, on the same connection, to issue 'SET enable_seqscan TO
false;' trough jdbc before I call my function, but execution time is
still around 400ms.

Mike