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Oracle Tips by Burleson 

Active Session History

Active Session History is known by its acronym ASH.  The ASH consists of session data that is sampled every second and stored in a circular buffer in System Global Areas (SGA).  Any session that is connected to the database is considered an active session as long as it is not waiting for an event that belongs to the class of waits defined as idle.

Unlike many of the acronyms in 10g, the “A” in ASH stands for “Active” rather than “Automatic”.  Tuning with ASH involves using the v$active_session_history view and workload repository tables such as wrh$active_session_history.  The workload repository is discussed in the next section in this chapter.  The ASH structures store the history of a session’s recent activity, including wait events.  The ASH allows analysis of the system performance at the current time as well as in recent history.  ASH is designed as a rolling buffer in memory, and earlier information is overwritten when needed.  ASH uses memory of the System Global Area (SGA).  Figure 8.1 shows the structure of v$active_session_history.

SQL> desc v$active_session_history

 Name                       Null?      Type

 -------------------------- ---------- -----------------

SAMPLE_ID   NUMBER

SAMPLE_TIME TIMESTAMP(3)

SESSION_ID  NUMBER

SESSION_SERIAL#   NUMBER

USER_ID     NUMBER

SQL_ID      VARCHAR2(13)

SQL_CHILD_NUMBER  NUMBER

SQL_PLAN_HASH_VALUE     NUMBER

SQL_OPCODE  NUMBER

SERVICE_HASH      NUMBER

SESSION_TYPE      VARCHAR2(10)

SESSION_STATE     VARCHAR2(7)

QC_SESSION_ID     NUMBER

QC_INSTANCE_ID    NUMBER

EVENT VARCHAR2(64)

EVENT_ID    NUMBER

EVENT#      NUMBER

SEQ#  NUMBER

P1    NUMBER

P2    NUMBER

P3    NUMBER

WAIT_TIME   NUMBER

TIME_WAITED NUMBER

CURRENT_OBJ#      NUMBER

CURRENT_FILE#     NUMBER

CURRENT_BLOCK#    NUMBER

PROGRAM     VARCHAR2(48)

MODULE      VARCHAR2(48)

ACTION      VARCHAR2(32)

CLIENT_ID   VARCHAR2(64)


The above book excerpt is from:

Oracle Wait Event Tuning

High Performance with Wait Event Iinterface Analysis 

ISBN 0-9745993-7-9  

Stephen Andert 

http://www.rampant-books.com/book_2004_2_wait_tuning.htm

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