PostgreSQL is an advanced Object-Relational database management system (DBMS). The postgresql packages include the client programs and libraries needed to access a PostgreSQL DBMS server.
Red Hat Enterprise Linux 6 features version 8.4 of PostgreSQL
MySQL is a multi-user, multi-threaded SQL database server. It consists of the MySQL server daemon (mysqld) and many client programs and libraries.
This release features version 5.1 of MySQL. For a list of all enhancements that this version provides, refer to the MySQL Release Notes