Oracle collects a large amount of statistical data regarding performance and activity. This data accumulates in memory and is then written periodically to disk in what is known as a Snapshot. This information is stored in a set of tables called the Automatic Workload Repository (AWR). The AWR is created in the SYSAUX Tablespace, and its location cannot be changed. It belongs to the SYSMAN user. While you can access the database through the SYSMAN user, you cannot access the AWR tables because Oracle does not support access to these tables through SQL*Plus.

Generally, the easiest way to access AWR information is through Enterprise Manager.

The statistics accumulate in the SGA memory before being sent to disk by the MMON process. By default, the Snapshot is sent to disk every hour, and it is retained on disk for seven days. However, this configuration can generally be changed. The collection of statistics is controlled by the STATISTIC_LEVEL variable, which must take one of the values ALL or TYPICAL. By default, this variable takes the value TYPICAL.

Snapshots are deleted after a defined period of time, typically seven days. However, in some cases, you may need to retain the Snapshots for a longer period for various reasons. In such cases, the Preserved Snapshot Sets option is used to specify which Snapshots you wish to keep for a longer period.

Of course, creating a Preserved Snapshot means that you can compare performance now with performance during a specific period in the past. These reports are retained for 30 days by default, but if you create a Preserved Snapshot, you can extract reports at any time.