Awful FATAL ERROR running query

I have a problem with postgres, I'm running a query that joins two tables,
one with 129000+ records, and the other with 1172 records, it's a very
simple join. But postgres can't handle it!!! (Both tables have an index on
the field "cuentacb").

I get the following error after 3 minutes...

migracion=# select * from cuentasequivalentes, equiv11mayo m
migracion-# where cuentasequivalentes.cuentacb = m.cuentacb
migracion-# and cuentasequivalentes.cuentabr <> m.cuentabr;
FATAL 1: Memory exhausted in AllocSetAlloc()
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.

What can I do? I'm guessing that I could change a configuration parameter or
something like that, but I don't know.

Please help me, it's kind of urgent.

Ligia