Fata error 1?!

6.5.3 for Red Hat Linux 6.0, RAM is 128 MB, IBM-PC.

start 'postmaster' by using 'nohup postmaster -i -d > pgserver.log 2>&1
&'.

A table 'tab_a' has about 100000 rows. when I do 'select count(*) from
tab_a;', I got an error as following and be kicked out of 'psql':

pqReadData()--backend closed the channel unexpectally. This probably
means the back end terminates abnormally before or while processing the
request. We have lost the connection to backend, so further processing
is impossible. Terminating.

I also check the 'pgserver.log' file, it shows:

FATA1: memeory exhausted in AllocSetAlloc()
proc_exit(0) [#0]
shmem_exit(0) [#0]
exit(0)
/usr/local/pgsql/bin/postmaster: reaping dead process.....
/usr/local/pgsql/bin/postmaster: CleanupProc: pid 670 exit with status
0.

I restart postmaster with '-B 3096' option to assign more buffers to
postgreSQL, but I got the same error.

Could anyone help me out? And one suggestion: we really should create a
news group for PostgreSQL!!!!!!

Thanks all,
Jack