Overview of Active Session History

To capture a detailed history of database activity, Oracle Database samples active sessions each second with the Active Session History (ASH) sampler. AWR snapshot processing collects the sampled data into memory and writes it to persistent storage. ASH is an integral part of the Oracle Database self-management framework and is extremely useful for diagnosing performance problems.

ASH gathers sampled data at the session level rather than at the instance level. By capturing statistics for only active sessions, ASH collects a manageable set of data. The size of this data is directly related to the work being performed, rather than to the size of the entire database instance.

Sampled data captured by ASH can be aggregated based on the dimensions in the data, including the following:

SQL identifier of a SQL statement
Object number, file number, and block number
Wait event identifier and parameters
Session identifier and session serial number
Module and action name
Client identifier of the session
Service hash identifier

You can run ASH reports to analyze transient performance problems with the database that only occur during specific times. This technique is especially useful when you are trying to do either of the following:

Resolve transient performance problems that may last for only a short period of time, such as why a particular job or session is not responding when the rest of the instance is performing as usual
Perform scoped or targeted performance analysis by various dimensions or their combinations, such as time, session, module, action, or SQL identifier