The Oracle 10g Database introduces a new component called Active Session History (ASH) which can be used to identify blocker and waiter session identities and their associated transaction identifiers and SQL for a specified duration of the report. This data for the sample session activity in the instance is provided by V$ACTIVE_SESSION_HISTORY view. A circular buffer in the SGA contains the sampled data of the active sessions. All sessions that are connected to the db and are waiting for an non-idle class event is considered an active session. This includes any session that was running on the CPU at the time of sampling. Sampling is done every second and contains a lot of useful information for Oracle Performance Tuning

The system view V$ACTIVE_SESSION_HISTORY returns one row for each active session per sample, returning the latest session sample rows first. The duration for which data is stored in Oracle 10g Data Dictionary is based fully on the database activity due to the circular nature of the buffer containing this information.

By capturing only active sessions, a manageable set of data is represented with the size being directly related to the work being performed rather than the number of sessions allowed on the system. Using the Active Session History enables you to examine and perform detailed analysis on both current data in the V$ACTIVE_SESSION_HISTORY view and historical data in the DBA_HIST_ACTIVE_SESS_HISTORY view, often avoiding the need to replay the workload to gather additional performance tracing information. The data present in ASH can be rolled up on various dimensions that it captures, including the following: