Answer

The management of the pg.log is governed by /etc/logrotate.d/netezza which is defined as follows:

 /nz/kit/log/postgres/pg.log {
    size 1024M
    missingok
    postrotate
        pkill -HUP -f postmaster 2>/dev/null || true
    endscript
}


These settings cause the pg.log file to rotate out when it has reached 1 GB in size.  If necessary, you can modify the attributes to rotate the log daily, or when it reaches a smaller size.  

To rotate the Postgres log daily and only keep four weeks of pg.logs online, modify the/etc/logrotate.d/netezza file to read as follows:

/nz/kit/log/postgres/pg.log {
    size 1024M
    daily
    rotate 28
    missingok
    postrotate
        pkill -HUP -u nz -f postmaster 2>/dev/null || true
    endscript
}


'daily' instructs Postgres to rotate the file daily in addition to when the size reaches 1GB.  'rotate 28' instructs it to keep 28 rotated logs online -- or 4 weeks.

The logrotate command is supplied with Red Hat Linux.  You can learn more about this command from the Red Hat manual page by running the following command: 
man logrotate 

Also, if you find you need to force the pg.log to rotate, you can issue the following command:
/usr/sbin/logrotate -f /etc/logrotate.d/netezza

DO NOT JUST COPY THE ACTIVE PG.LOG TO ANOTHER LOCATION ON A RUNNING SYSTEM.