-- versions 7.4 thru 9:
# select substring(version() from E'^PostgreSQL (\\S+)') ; 
 substring 
-----------
 8.4.4
(1 row)

-- versions 7.4 thru 9:
# select current_setting('server_version'); 
 current_setting 
-----------------
 8.4.4
(1 row)

-- versions 8.2 thru 9:
# select current_setting('server_version_num');
 current_setting 
-----------------
 80404
(1 row)