http://httpd.apache.org/docs/2.0/ssl/ssl_faq.html#comp
That applies in general to SSL apps.
In cryptography it is always recommended, and sometimes even mandatory
to compress data before encryption. This reduces the risk of finding
patterns, etc.
And SSL includes that option as well.
But that's as far as my knowledge goes. Whether postgresql can utilize
it, and how to set it up with postgresql - dunno.
--
GJ