Another option, which focuses more on the longer term, can be performed during a maintenance opportunity. This is to move high-concurrency segments into a locally managed tablespace with ASSM enabled. ASSM does not use free lists, but instead uses bitmaps to manage free space. While this usually allows for increased table segment header block concurrency, I have spoken with a few of my students who have unexpectedly experienced increased buffer busy activity when converting to ASSM. For some reason (and they never found out), an unknown number of events (for example, workload, Oracle release, application design) had contributed to stressing ASSM in an unexpected way.

Undo segments are different from table segments because they contain information related to transaction rollback and read consistency. One of the Oracle structures central for rollback and read consistency to properly occur is called the transaction table. Simply put, the transaction table is a map to an undo segment's contents. Every undo segment contains a single transaction table, which is located in the undo segment's header block. When intense DML, especially when combined with read consistency activity, occurs, the transaction table can become the point of contention, resulting in buffer busy waits events on an undo segment header block. Before moving into how to solve this issue, an introduction to the transaction table is warranted.

By default, every Oracle transaction generates redo (roll forward) and undo (roll back) information, as well as the actual data change and the possible index changes. The undo information is stored in an undo segment, and the map of each undo segment is stored in its transaction table. Each undo segment's transaction table can hold multiple transaction entries. From a relational database way of thinking, each entry corresponds to a row in the transaction table. Having the word table as part of its name is unfortunate, because the transaction table is not a relational database structure. In fact, the Oracle kernel developers refer to the transaction table entries as slots.