Overview

A wait event is a database event indicating that something is waiting for something else. In other words, a process could be waiting for another process to release a resource that the first process requires.

A wait event occurs because something is being processed where other processes waiting for its completion are dependent on the result of the event causing the wait. When one process waits for another to complete a wait event is recorded in database statistics.

Much like ratio calculations wait events are sometimes best analyzed as symptoms of problems, such as contention. Wait event frequency points to symptoms of problems, rather than being direct solutions. Therefore, tune the problem causing the wait event. Do not necessarily simply attempt to reduce wait events without being sure that the real cause of the problem is addressed, although sometimes this will be the case. Understand the problem. Also note that frequent occurrence of some wait events is normal, depending on database type and applications.

There are numerous performance views involved in recording of wait event details. These are some of them:

• V$EVENT_NAME. All possible events.

• V$SYSTEM_EVENT. Total waits for all occurring database events, aggregated for all sessions.

• V$SESSION_EVENT. The same information as V$SYSTEM_ EVENT except detailing events per session rather than aggregated for all sessions.

• V$SESSION_WAIT. This view is session level and shows a real-time picture where wait events are listed as they are happening. Contains hooks through to...