Active Session History Statistics
The Active Session History (ASH) statistics are samples of session activity in the database. The database samples active sessions every second and stores them in a circular buffer in the System Global Area (SGA). Any session that is connected to the database and using CPU, or is waiting for an event that does not belong to the idle wait class, is considered an active session. By capturing only active sessions, a manageable set of data is represented. The size of the data is directly related to the work being performed, rather than the number of sessions allowed on the database.
Using the DB time example described in “Time Model Statistics”, samples of session activity are collected from the online transaction made at the bookseller’s website, represented as vertical lines below the horizontal arrow in Figure 2-3.
Figure 3: Active Session History
Description of the illustration tdppt003.gif
The light vertical lines represent samples of inactive session activity that are not captured in the ASH statistics. The bold vertical lines represent samples of active sessions that are captured at:
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7:38, while novels by the author are being queried
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7:42, while the user is browsing the query results
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7:50, when one novel is added to the shopping cart
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7:52, during the checkout process
Table 2-1 lists ASH statistics collected for the active sessions, along with examples of the session ID (SID), module, SQL ID, session state, and wait events that are sampled.
Table 1 Active Session History
| Time | SID | Module | SQL ID | State | Event |
|---|---|---|---|---|---|
| 7:38 | 213 | Book by author | qa324jffritcf | Waiting | db file sequential read |
| 7:42 | 213 | Get review ID | aferv5desfzs5 | CPU | n/a |
| 7:50 | 213 | Add item to cart | hk32pekfcbdfr | Waiting | buffer busy wait |
| 7:52 | 213 | Checkout | abngldf95f4de | Waiting | log file sync |