The function used compute the amount increase each row data

CHAPTER 16 �� INSTANCE TUNING 367

In Listing 16-4, you see another valuable SQL query; it provides the distribution of DB time for each available time period. It is a fairly long query, but you can download the text from the Source Code section of the Apress web site (��). You have to specify a range of snapshots; the appropriate range can be obtained from the �� table. As I explained earlier in this chapter, the distribution of DB time is precisely the information you need to tune the database. CPU time is obtained from the �� table, while the wait times for each type of event are obtained from the �� table. The classifications in the �� view are used to group events into categories such as Application, Concurrency, Commit, User IO, and System IO. The �� operator is used to convert rows into columns; this makes it easy to perform further analysis on the data. The �� function is used to compute the amount of increase in each row of data. Finally, we divide CPU time and wait time by the length of the observed interval to compute average active sessions for each category of time.

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368 CHAPTER 16 �� INSTANCE TUNING

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