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CHAPTER 20 PERFOR MAN CE TUNING: TUNING THE INSTA NCE
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WAIT_CLASS#: This is the number of the wait class. WAIT_CLASS: This is the name of the wait class. WAIT_TIME: This is the wait time in seconds if the state is waited known time. SECONDS_IN_WAIT: This is the wait time in seconds if the state is waiting. STATE: The state could be waited short time, waited known time, or waiting, if the session is waiting for an event. The fourth wait-related view is the V$SESSION view. Not only does this view provide many details about the session, it also provides significant wait information as well. The V$SESSION view contains all the columns of the V$SESSION_WAIT view, plus a number of other important session-related columns. Because of this overlap of wait information in the V$SESSION and the V$SESSION_WAIT views, you can use the V$SESSION view directly to look for most of the wait-related information, without recourse to the V$SESSION_WAIT view. You can start analyzing the wait events in your system by first querying the V$SYSTEM_EVENT view to see if any significant wait events are occurring in the database. You can do this by running the query shown in Listing 20-15. Listing 20-15. Using the V$SYSTEM_EVENT View to View Wait Events SQL> 2 3 4* SELECT event, time_waited, average_wait FROM V$SYSTEM_EVENT GROUP BY event, time_waited, average_wait ORDER BY time_waited DESC; TIME_WAITED -----------24483121 18622096 12485418 3120909 3093214 3024203 831831 107253 52955 19958 5884 AVERAGE_WAIT --------------216.71465 106.19049 205.01844 306.93440 29459.18100 1536.68852 .25480 .90554 43.08787 2.02639 1.47505
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EVENT ---------------------------rdbms ipc message SQL*Net message from client PX Idle Wait pmon timer smon timer PL/SQL lock timer db file sequential read db file scattered read free buffer waits log file parallel write latch free . . . 58 rows selected. SQL>
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This example shows a simple system with hardly any waits other than the idle type of events and the SQL*Net wait events. There aren t any significant I/O-related or latch-contention related wait events in this database. The db file sequential read (caused by index reads) and the db file scattered read (caused by full table scans) wait events do seem somewhat substantial, but if you compare the total wait time contributed by these two events to the total wait time since the instance started, they don t stand out. Furthermore, the AVERAGE_WAIT column shows that both these waits have a low average wait time (caused by index reads). I discuss both these events, along with several other Oracle wait events, later in this chapter, in the section Important Oracle Wait Events. However, if your query on a real-life production system shows significant numbers for any nonidle wait event, it s probably a good idea to find out the SQL statements that are causing the waits. That s where you have to focus your efforts to reduce the waits. You have different ways to obtain the associated SQL for the waits, as explained in the following section.
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CH A PT ER 2 0 PERF O RMAN CE TUNI NG: TUN ING TH E I NS TA NCE
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Obtaining Wait Information
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Obtaining wait information is as easy as querying the related dynamic performance tables. For example, if you wish to find out quickly the types of waits different user sessions (session-level wait information) are facing and the SQL text of the statements they re executing, you can use the following query: SQL> 2 3 4 5 6 7* SELECT s.username, t.sql_text, s.event FROM V$SESSION s, V$SQLTEXT t WHERE s.sql_hash_value = t.hash_value AND s.sql_address = t.address AND s.type <> 'BACKGROUND' ORDER BY s.sid,t.hash_value,t.piece;
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You need to turn on statistics collection by either setting the initialization parameter TIMED_STATISTICS to TRUE or setting the initialization parameter STATISTICS_LEVEL to TYPICAL or ALL.
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If you want a quick instance-wide wait event status, showing which events were the biggest contributors to total wait time, you can use the query shown in Listing 20-16 (several idle events are listed in the output, but I don t show them here). Listing 20-16. Instance-Wide Waits Sorted by Total Wait Time SQL> 2 3 4 5 6* SELECT event, total_waits,time_waited FROM V$SYSTEM_EVENT WHERE event NOT IN ('pmon timer','smon timer','rdbms ipc reply','parallel deque wait','virtual circuit','%SQL*Net%','client message','NULL ORDER BY time_waited DESC; TOTAL_WAITS ----------35051309 1373973 2958367 2837 444743 146221 TIME_WAITED -----------15965640 1913357 1840810 370871 252664 123435
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event')
EVENT -----------------------db file sequential read latch free db file scattered read enqueue buffer busy waits log file parallel write SQL>
The preceding query shows that waits due to the db file scattered read wait event account for most of the waits in this instance. The db file sequential read wait event, as you ll learn shortly, is caused by full table scans. It s somewhat confusing in the beginning when you re trying to use all the wait-related V$ views, which all look similar. Here s a quick summary of how you go about using the key wait-related Oracle Database 11g dynamic performance views. First, look at the V$SYSTEM_EVENT view and rank the top wait events by the total amount of time waited, as well as the average wait time for that event. Start investigating the top waits in terms of the percentage of total wait time. You can also look at any AWR reports you may have, because the AWR also lists the top five wait events in the instance. Next, find out more details about the specific wait event that s at the top of the list. For example, if the top event is buffer busy waits, look in the V$WAITSTAT view to see which type of buffer block (data block, undo block, and so on) is causing the buffer busy waits (a simple SELECT * from V$WAITSTAT
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