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CHAPTER 22 PERFORMANCE TUNING: TUNING THE INSTANCE
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Listing 22-15. 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;
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EVENT TIME_WAITED AVERAGE_WAIT -----------------------------------------------------------------rdbms ipc message 24483121 216.71465 SQL*Net message from client 18622096 106.19049 PX Idle Wait 12485418 205.01844 pmon timer 3120909 306.93440 smon timer 3093214 29459.18100 PL/SQL lock timer 3024203 1536.68852 db file sequential read 831831 .25480 db file scattered read 107253 .90554 free buffer waits 52955 43.08787 log file parallel write 19958 2.02639 latch free 5884 1.47505 ... 58 rows selected. SQL> 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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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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CHAPTER 22 PERFORMANCE TUNING: TUNING THE INSTANCE
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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 22-16 (several idle events are listed in the output, but I don t show them here). Listing 22-16. Instance-Wide Waits Sorted by Total Wait Time SQL> 2 3 4 5 6* SELECT event, total_waits,time_waited FROM V$SYSTEM_EVENT WHERE event NOT IN ('pmon timer','smon timer','rdbms ipc reply','parallel deque wait ,'virtual circuit','%SQL*Net%','client message','NULL ORDER BY time_waited DESC;
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event')
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EVENT TOTAL_WAITS TIME_WAITED -----------------------------------------------------------------db file sequential read 35051309 15965640 latch free 1373973 1913357 db file scattered read 2958367 1840810 enqueue 2837 370871 buffer busy waits 444743 252664 log file parallel write 146221 123435 SQL> The preceding query shows that waits due to the db file scattered read wait event account for most of the waits in this instance. The db file sequential read wait event, as you ll learn shortly, is caused by full table scans. It s somewhat confusing in the beginning when you re trying to use all the wait-related V$ views, which all look similar. Here s a quick summary of how you go about using the key wait-related Oracle Database 10g 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$WAITSTAT gets you all the necessary information). For example, if the undo-block buffer waits make up most of your buffer busy waits, then the undo segments are at fault, not the data blocks. Finally, use the V$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 STATE -----WAITING EVENT ----------------------db file sequential read WAIT_TIME --------0 SECONDS_IN_WAIT --------------0
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