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CHAPTER 22 PERFORMANCE TUNING: TUNING THE INSTANCE
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Configuration System I/O Administrative Scheduler 11 rows selected. SQL>
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The V$SESSION_WAIT_CLASS view shows the total time spent in each type of wait class by an individual session. Here s an example: SQL> 2 3 4* SELECT wait_class, time_waited FROM v$session_wait_class WHERE sid = 1053 ORDER BY time_waited DESC;
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WAIT_CLASS TIME_WAITED -----------------------------------------Idle 21190 User I/O 8487 Other 70 Concurrency 13 Application 0 Network 0 6 rows selected. SQL> The V$WAITCLASSMETRIC view shows metric values of wait classes for the most recent 60-second interval. The view keeps information for up to one hour. Here s an example of using the query: SQL> 2 3 4* SELECT WAIT_CLASS#, WAIT_CLASS_ID dbtime_in_wait,time_waited,wait_count FROM v$waitclassmetric ORDER BY time_waited DESC;
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WAIT_CLASS# DBTIME_IN_WAIT TIME_WAITED WAIT_COUNT ----------- -------------- ----------- ---------6 2723168908 170497 51249 0 1893977003 5832 58 8 1740759767 717 1351 5 3386400367 11 68 7 2000153315 8 52906 9 4108307767 6 99 1 4217450380 0 4 2 3290255840 0 0 3 4166625743 0 0 11 3871361733 0 0 10 2396326234 0 0 4 3875070507 0 0 12 rows selected. SQL> As you can see, WAIT_CLASS 6 tops the list, meaning that idle class waits currently account for most of the wait time in this instance.
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CHAPTER 22 PERFORMANCE TUNING: TUNING THE INSTANCE
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Looking at Segment-Level Statistics
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Whether you use the AWR or the wait-related V$ views, you re going to find no information about where a certain wait event is occurring. For example, you can see from the V$SYSTEM_EVENT view that buffer busy waits are your problem, and you know that you reduce these waits by switching from manual segment space management to Automatic Segment Space Management (ASSM). However, neither AWR nor the V$ view indicates which tables or indexes you should be looking at to fix the high wait events. Oracle provides three V$ views to help you drill down to the segment level. The segment-level dynamic performance views are V$SEGSTAT_NAME, V$SEGSTAT, and V$SEGMENT_STATISTICS. Using these, you can find out which of your tables and indexes are being subjected to high resource usage or high waits. Once you re aware of a performance problem due to high waits, you can use these segment-level views to find out exactly which table or index is the culprit and fix that object to reduce the waits and increase database performance. The V$SEGMENT_ NAME view provides you with a list of all the segment levels that are being collected, and tells you whether the statistics are sampled or not. Let s see how you can use these segment-level views to your advantage when you re confronted with a high number of wait events in your system. Say you look at the V$SYSTEM_EVENT view and realize that there are a large number of buffer busy waits. You should now examine the V$SEGMENT_STATISTICS view with a query such as the following to find out which object is the source of the high buffer busy waits. You can then decide on the appropriate corrective measures for this wait event, as discussed in the section Important Oracle Wait Events later in this chapter. SQL> 2 3 4* SELECT owner, object_name, object_type, tablespace_name FROM V$SEGMENT_STATISTICS WHERE statistic_name='buffer busy waits' ORDER BY value DESC; OBJECT_NAME -------------LAB_DATA LAB_ADDR_I PERS_SUMMARIES OBJECT_TYPE ----------TABLE INDEX TABLE TABLESPACE_NAME ----------------LAB_DATA_D LAB_DATAS_I PERS_SUMMARIES_D
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OWNER --------SYSOWNER SYSOWNER SYSOWNER ... SQL>
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Collecting Detailed Wait Event Information
Selecting data from V$ dynamic performance views and interpreting them meaningfully isn t always so easy to do. Because the views are dynamic, the information that they contain is constantly changing as Oracle updates the underlying tables for each wait event. Also, the wait-related dynamic performance views you just examined don t provide crucial data such as bind variable information. For a more detailed level of wait information, you can use one of the methods described in the following sections.
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