We done performance benchmark on Oracle, SQLServer and DB2 database using the same version of application, tp, os, server and same set of performance data. There are very little difference between those databases. The more noticeable difference is the scalability. Oracle and DB2 are more scalable.

However, there is one thing people always overlook. DB2 has more overhead in maintenance. You need to compile all your programs as packages and create the package in the database. If there is any change in the program code, you need to recompile and recreate the DB2 package. The package is how DB2 improves its performance.

So I still prefer Oracle over DB2 and SQLServer is not on my list as it does not run on Unix.