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CHAPTER 20 PERFOR MAN CE TUNING: TUNING THE INSTA NCE
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gets you all the necessary information). For example, if the undo-block buffer waits make up most of your buffer busy waits, then the undo segments are at fault, not the data blocks. Finally, use the V$SESSION view to find out the exact objects that may be the source of a problem. For example, if you have a high amount of db file scattered read type waits, the V$SESSION view will give you the file number and block number involved in the wait events. In the following example, the V$SESSION view is used to find out who is doing the full table scans showing up as the most important wait events right now. As explained earlier, the db file scattered read wait event is caused by full table scans. SQL> SELECT sid, sql_address, sql_hash_value FROM V$SESSION WHERE event = 'db file scattered read'; Here s an example that shows how to find out the current wait event for a given session: SQL> SELECT sid, state, event, wait_time, seconds_in_wait 2 FROM v$session 3*WHERE sid=1418; SID --1418 SQL> STATE -----WAITING EVENT ----------------------db file sequential read WAIT_TIME --------0 SECONDS_IN_WAIT --------------0
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The value of 0 under the WAIT_TIME column indicates that the wait event db file sequential read is occurring for this session. When the wait event is over, you ll see values for the WAIT_TIME and the SECONDS_IN_WAIT columns. You can also use the V$SQLAREA view to find out which SQL statements are responsible for high disk reads. If latch waits predominate, you should be looking at the V$LATCH view to gain more information about the type of latch that s responsible for the high latch wait time: SQL> 2 3 4* SID ---1218 1400 SQL> SELECT sid, blocking_session, username, event, seconds_in_wait siw FROM V$SESSION WHERE blocking_session_status = 'VALID'; BLOCKING_SESS ------------1527 1400 USERNAME -------UCR_USER APPOWNER EVENT ----------------------------enq: TX - row lock contention latch free SIW ----23 0
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The V$SESSION_WAIT_HISTORY View
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The V$SESSION_WAIT_HISTORY view holds information about the last ten wait events for each active session. The other wait-related views, such as the V$SESSION and the V$SESSION_WAIT, show you only the wait information for the most recent wait. This may be a short wait, thus escaping your scrutiny. Here s a sample query using the V$SESSION_WAIT_HISTORY view: SQL> 2 3 4* SELECT seq#, event, wait_time, p1, p2, p3 FROM V$SESSION_WAIT_HISTORY WHERE sid = 988 ORDER BY seq#;
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SEQ# EVENT -------------------------1 db file sequential read 2 db file sequential read 3 latch: shared pool 4 db file sequential read 5 db file sequential read 6 db file sequential read 7 db file sequential read 8 db file sequential read 9 db file sequential read 10 db file sequential read 10 rows selected. SQL>
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WAIT_TIME P1 --------- ----0 0 0 1.3835E+19 0 4 1 2 2 0 0
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P2 P3 ----- -----52 21944 50 19262 198 0 205 21605 52 13924 49 29222 52 14591 52 12723 205 11883 205 21604
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Note that a zero value under the WAIT_TIME column means that the session is waiting for a specific event. A nonzero value represents the time waited for the last event.
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Analyzing Waits with Active Session History
The V$SESSION_WAIT view tells you what resource a session is waiting for. The V$SESSION view also provides significant wait information for active sessions. However, neither of these views provides you with historical information about the waits in your instance. Once the wait is over, you can no longer view the wait information using the V$SESSION_WAIT view. The waits are so fleeting that by the time you query the views, the wait in most times is over. The new Active Session History feature, by recording session information, enables you to go back in time and review the history of a performance bottleneck in your database. Although the AWR provides hourly snapshots of the instance by default, you won t be able to analyze events that occurred five or ten minutes ago, based on AWR data. This is where the ASH information comes in handy. ASH samples the V$SESSION view every second and collects the wait information for all active sessions. An active session is defined as a session that s on the CPU or waiting for a resource. You can view the ASH session statistics through the view V$ACTIVE_SESSION_HISTORY, which contains a single row for each active session in your instance. ASH is a rolling buffer in memory, with older information being overwritten by new session data. Every 60 minutes, the MMON background process flushes filtered ASH data to disk, as part of the hourly AWR snapshots. If the ASH buffer is full, the MMNL background process performs the flushing of data. Once the ASH data is flushed to disk, you won t be able to see it in the V$ACTIVE_ SESSION_HISTORY view. You ll now have to use the DBA_HIST_ACTIVE_SESS_HISTORY view to look at the historical data. In the following sections, I show how you can query the V$ACTIVE_SESSION_HISTORY view to analyze current (recent) Active Session History.
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