Urgent - Help - Required - Postgres - 6.5

Hi,

We are running a large postgres database on redhat linux 6.2 & we
have'nt changed any default settings of postgres,
Earlier the same database was running on free BSD(we don't know
the settings of earlier running postgres as we are
somewhat new to the administration part of postgres sql). We
created the new database on linux with that dump.

No when we take the dump of the existing database we get the
following error :


Backend message type 0x44 arrived while idle
dumpClasses(): command failed.  Explanation from backend: 'Backend
message type 0x44 arrived

while idle
'.

and when we query the table (having more than 17000 records) it
sometimes returns the correct result or gives the following errors
:

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

Terminating.


NOTICE:  Message from PostgreSQL backend:
         The Postmaster has informed me that some other backend
died abnormally and possibly

corrupted shared memory.
         I have rolled back the current transaction and am going
to terminate your database

system connection and exit.
         Please reconnect to the database system and repeat your
query.
pqFlush() -- backend closed the channel unexpectedly.
         This probably means the backend terminated abnormally
before or while processing the

request.

Kindly let me know the solution of the above problem.


More over I have 512 mb of ram on my database server can u please
tell me what shud be optimum value of the following variables :

shared_buffers
sort_mem
max_connections
fsync

and how to change the value as i'm using version 6.5 .


Thanks,

Neeraj
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