After researching all the available databases which are free and source code is available, it was found that ONLY PostgreSQL is the MOST mature, most widely used and robust RDBMS SQL free database (object relational) in the world.
PostgreSQL is very appealing since lot of work had already been done. It has ODBC and JDBC drivers, using these it is possible to write applications independent of the databases. The applications written in PostgreSQL using ODBC, JDBC drivers are easily portable to other databases like Oracle, Sybase and Informix and vice versa.
You may ask "But why PostgreSQL ?" The answer is, since it takes lot more time to develop a database system from scratch, it makes sense to pick up a database system which satisfies the following conditions -
A database system