Communication error while getting a tuple

Hi All,
I am connecting to postgres cluster using ODBC postgres driver and I am
getting error as

Description: 'Communication error while getting a tuple;

Error fetching next row'

SQLState: *08S01*(SQLSTATE_COMMUNICATION_LINK_FAILURE)

What can be possible reason for the same?

I am using the following script to get the tuples:

import pyodbc

cnxn = pyodbc.connect('DRIVER={PostgreSQL
UNICODE};SERVER=hostname;DATABASE=testdb;PORT=port;UID=test;PWD=password;UseDeclareFetch=1;Fetch=999;sslmode=prefer')

cursor = cnxn.cursor()
cursor.execute(sql)
ressize=0
while(True):
rows=cursor.fetchmany(999)

sz=len(rows)
ressize=ressize+sz

if sz == 0:
break;
print "read =" + str(ressize)

The result size for a query is huge around 10M rows.
Thanking you.

- pguser