Statistics and Wait Events

    Statistics are counters of events that happen in the database. These are indicators of the amount of useful work that is being completed. Statistics include counts such as the number of user commits or db file sequential reads. Many statistics have corresponding wait events. There are several statistics that can indicate performance problems that do not have corresponding wait events. The full list of statistics can be found in the V$STATNAME view.

  Wait events are statistics that are incremented by a server process or thread to indicate that it had to wait for an event to complete before being able to continue processing. Wait event data reveals various symptoms of problems that might be affecting performance, such as latch contention, buffer contention, and I/O contention.

  Remember that these are only symptoms of problems, not the actual causes. The full list of wait events can be found in the V$EVENT_NAME view.

  The CLASS column for each statistic contains a number representing one or more statistic classes. The following class numbers are additive:

1, User
2, Redo
4, Enqueue
8, Cache
16, OS
32, Real Application Clusters
64, SQL
128, Debug

For example, a class value of 72 represents a statistic that relates to SQL statements and caching.


Note: Some statistics are populated only if the TIMED_STATISTICS initialization parameter is set to true.

Wait Classes