# Edit your /etc/postgresql/9.3/main/postgresql.conf, and change the lines as follows:

# Note: If you didn't find the postgresql.conf file, then just type 

$> locate postgresql.conf 

# in a terminal

1) change #log_directory = 'pg_log' to log_directory = 'pg_log'
2) change #log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log' to log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log'
3) change #log_statement = 'none' to log_statement = 'all'
4) change #logging_collector = off to logging_collector = on

# Optional: SELECT set_config('log_statement', 'all', true);

sudo /etc/init.d/postgresql restart or sudo service postgresql restart

#Fire query in postgresql: select 2+2

# Find current log in /var/lib/pgsql/9.2/data/pg_log/

#The log files tend to grow a lot over a time, and might kill your machine. For your safety, write a bash script that'll delete logs and restart postgresql server.