hash join --- hash table out of memory

Hi:
 I had two tables tenk1 and tenk2. Tenk1 contains 25000 tuples. Tenk2
contains 30000 tuples. I use "postmaster -S -o'-fm -fn'" to start
postmaster. Then ,when I execute the following query:
    select count(*) from tenk1,tenk2 where
tenk1.unique1=tenk2.unique2;
it reports:
     hash table out of memory. Use -B parameter to increase buffers.
Then I use -B256 to increase buffers. But it also report the error
messages. Is there any effort taken to avoid such errors?
Thanks in advance!
S.Q.Yang