Wait event - DB CAFE

Wait Events Wait events are statistics that are incremented by a server process/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 impacting performance, such as latch contention, buffer contention, and I/O contention.

To enable easier high-level analysis of the wait events, the events are grouped into classes. The wait event classes include: Administrative, Application, Cluster, Commit, Concurrency, Configuration, Idle, Network, Other, Scheduler, System I/O, and User I/O.

The wait classes are based on a common solution that usually applies to fixing a problem with the wait event. For example, exclusive TX locks are generally an application level issue and HW locks are generally a configuration issue.

The following list includes common examples of the waits in some of the classes:

Application: locks waits caused by row level locking or explicit lock commands

Commit: waits for redo log write confirmation after a commit

Idle: wait events that signify the session is inactive, such as SQL*Net message from client

Network: waits for data to be sent over the network

User I/O: wait for blocks to be read off a disk

Wait event statistics for an instance include statistics for both background and foreground processes. Because you would typically focus your effort in tuning foreground activities, overall instance activity is broken down into foreground and background statistics in the relevant V$ views to facilitate tuning.

The V$SYSTEM_EVENT view provides wait event statistics for the foreground activities of an instance and the wait event statistics for the instance as a whole. The V$SYSTEM_WAIT_CLASS view provides these wait event statistics (foreground and instance statistics) after aggregating to wait classes.

The V$SESSION_EVENT and V$SYSTEM_WAIT_CLASS views provide wait event and wait class statistics at session level.