PostgreSQL defeated Oracle 8 (and 8i), IBM DB2, MS SQL server, Sybase, Interbase and MySQL in standard benchmark tests in performance, speed, scalability and reliability! Read the benchmarks at http://www.aldev.8m.com or at http://aldev.webjump.com
MySQL is another open-source SQL server, but it does not support transactions. It is suitable for very small databases and does not support advanced SQL functionalities. Whereas PostgreSQL is an enterprise strength database supporting transactions and almost all SQL constructs. PostgreSQL is much more advanced than commercial databases like Oracle, Sybase and Informix. PostgreSQL supports very advanced locking mechanisms and many more advanced features which are not available in commercial database systems!!
In near future development of MySQL will be dropped, since MySQL is duplicate product working towards ANSI SQL. We would take the most advanced and mature open-source SQL server and drop all others as we do not have lots of time (to deal with multiple RDBMSes)!! In fact, you do not have time to deal with just one powerful SQL server like PostgreSQL! And all the MySQL users will be migrated to PostgreSQL. Also MySQL is a 'quasi-commercial' product unlike PostgreSQL which is open-source and there is no license fee. There is no need for another SQL database system as PostgreSQL is already here in this world!!
Duplicate products like MySQL confuse the user base and causes division of resources. For a "NEAR PERFECT" system there must be only one system and everybody in the world must work on it!! Duplicate products cause more harm than good and hence division of resources must be strongly discouraged. This already happened in case of commercial database systems like Oracle, Sybase, Informix and MS SQL server which caused splintering of user base and often they are incompatible. I want put the source code of SQL server under your control!!!
You do not need hundreds of database systems, all you need is just one best database server which happens to be 'PostgreSQL'.
WARNING: It is possible to create infinite number of database systems for a given specification like ANSI SQL!!
Features which are missing in MySQL and which PostgreSQL supports are -
MySQL is at http://www.tcx.se
PostgreSQL should be compared with systems like Oracle, both are really true ACID compliant robust systems developed over a very long time. It is very much wrong to compare MySQL with Oracle or MySQL with PostgreSQL. For more details read Why Not MySQL. Hence, it will be a very serious mistake to replace Oracle with MySQL!! If you want to replace Oracle then consider PostgreSQL.