psycopg2 cursor hangs up when query time is too long

I have a problem with executing long time queries using psycopg2 in Python. When query takes more than 180 seconds the script execution hangs up for a long time.

I use Python 3.4.3 and psycopg2 2.6.1.

Here are samples to reproduce the issue:

import psycopg2

cnn = psycopg2.connect(
    database='**********',
    user='**********',
    password='**********',
    host='**********',
    port=5432,
)
print("Connected")
cursor = cnn.cursor()
seconds = 5
print("Sleep %s seconds"%seconds)
cursor.execute("SELECT pg_sleep(%s);"%seconds)
print("Exit.")

Script works fine when query takes 5 seconds:

$python3 /tmp/test.py 
Connected
Sleep 5 seconds
Exit.

But when number of seconds is about 180 and greater, the line cursor.execute hangs up and instructions below are never executed:

import psycopg2

cnn = psycopg2.connect(
    database='**********',
    user='**********',
    password='**********',
    host='**********',
    port=5432,
)
print("Connected")
cursor = cnn.cursor()
seconds = 180
print("Sleep %s seconds"%seconds)
cursor.execute("SELECT pg_sleep(%s);"%seconds)
print("Exit.")

Here is a output:

$python3 /tmp/test.py 
Connected
Sleep 5 seconds
<Never exit>

Does anyone know how to solve this problem? Thank you.

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