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The low-level information helps you zoom in on the root cause of performance problems The following columns from the V$SESSION_WAIT view are important for troubleshooting performance issues: EVENT: These are the different wait events described in the next section (for example, latch free and buffer busy waits) P1, P2, P3: These are the additional parameters that represent different items, depending on the particular wait event For example, if the wait event is db file sequential read, P1 stands for the file number, P2 stands for the block number, and P3 stands for the number of blocks If the wait is due to a latch free event, P1 stands for the latch address, P2 stands for the latch number, and P3 stands for the number of attempts for the event WAIT_CLASS_ID: Identifies the wait class WAIT_CLASS#: Number of the wait class WAIT_CLASS: Name of the wait class.

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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 waitrelated information, without recourse to the V$SESSION_WAIT view.

Summary

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 22-15..

172.16.5.1 node1 node1.somedomain.com alias1.somedomain.com 172.16.5.2 node2 node2.somedomain.com alias2.somedomain.com 172.16.5.14 node14 node14.newdomain.com alias14.newdomain.com 172.16.5.4 node3 node3.somedomain.com alias3.somedomain.com

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;

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.

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;

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.

This last example shows how to delete a line entirely. We search for a line containing the string node2 and delete it. This command deletes only the first line that is found in the file. A leading g on the command would implement the deletion for all lines found in the file. ed Command