-- 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)


##</code><code>##

perl -MDBI -e'my$dbh=DBI->connect("dbi:Pg:"); print $dbh->{pg_server_version}, "\n"';
80404

##</code><code>##

select  implementation_info_name, character_value 
  from  information_schema.sql_implementation_info
 where  implementation_info_name ~ 'DBMS';

 implementation_info_name | character_value 
--------------------------+-----------------
 DBMS NAME                | PostgreSQL
 DBMS VERSION             | 08.04.0004
(2 rows)