If the database is not using automatic undo management (AUM), and is therefore using traditional rollback segments, the solution is simple. Just create an additional rollback segment, which will also create an additional transaction table, thereby distributing transaction table activity. You will notice that the buffer busy wait contention subsides. Keep adding additional rollback segments until buffer busy waits is far from the top wait event.

Most Oracle systems today take advantage of Oracle AUM capabilities. By default, Oracle tries to assign only one active transaction per undo segment. If each undo segment has an active transaction and if there is space available in the undo tablespace, Oracle will automatically create an additional undo segment. This usually takes care of the buffer busy waits. However, if there is no more room in the undo segment tablespace, multiple transactions will be assigned to each undo segment, and eventually undo segment header contention will result. The solution is to add another database file to your undo segment tablespace, thus enabling Oracle to create additional undo segments. It's that simple.

Simply put, indexes are ordered structures. This ordering allows indexes to be used to quickly search. Any changes to the index must result in the order being maintained. If order is not maintained, searching could not occur quickly, and the index would be both worthless and corrupt. So order in an index must be maintained. This has profound performance implications.