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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 |
