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 impacting performance, such as latch contention, buffer contention, and I/O contention.
To enable easier high-level analysis of the wait events, events are grouped into classes.
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$SYSTEM_EVENT view shows wait event statistics for the foreground activities of an instance and the wait event statistics for the instance. The V$SYSTEM_WAIT_CLASS view shows these foreground and wait event instance statistics after aggregating to wait classes. V$SESSION_EVENT and V$SYSTEM_WAIT_CLASS show wait event and wait class statistics at the session level.
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