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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;

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.

For every type that introduces or overrides at least one virtual function, the compiler generates a vtable. For every virtual function that has been inherited or introduced, there is one pointer in the vtable. In the preceding code, Base does not inherit a virtual function, but it introduces f1 and f2. Therefore, its vtable has two pointers. Since Derived inherits two virtual functions (f1 and f2) and introduces a further one (f3), the Derived class s vtable has three pointers. Pointers in the vtable appear in the order in which they are declared, from the original base class via all other base classes to the class itself. This ensures that a vtable of a derived class starts with pointers for the same virtual functions as the vtable of its base class. These function pointers are either pointers to inherited virtual functions or pointers to overrides. Due to the compatibility of a vtable for a derived class with the vtable of its base class, a derived class s vtable can be treated as a specialization of its base class s vtable. Figure 6-1 shows two classes and their vtables.

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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.