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Latch wait events occur when a session needs to acquire a latch that is held by another session. Latches are simple, low-level serialization mechanisms to protect shared SGA data structures and shared code segments from simultaneous session access. They are similar in purpose to locks: latches protect internal memory structures while locks protect data structures. Latch wait events are designed to be very quickly acquired and freed.
The legal agreement that determines how many users can access each Oracle installation. See glossary definitions of Per-Seat Licensing and Per Server Licensing.
Lock wait events occur when a session attempts to acquire a lock that is already held by another session. A session will be blocked until the blocking session releases the lock. Locks are designed to ensure data integrity by limiting simultaneous data access.
Multi-user database locking generally consists of two levels: exclusive locks and share locks. You want to watch out for exclusive locks (that is, TX) as they prohibit resource sharing. For example, the first transaction that exclusively locks a resource is the only one that can alter the resource (except for the DBA) until the exclusive lock is released. Share locks, unlike exclusive locks, allow a resource to be shared.
Most locking issues are application-specific and can be addressed by tuning the concurrency logic in the application.
The number of data blocks read by Oracle, which were already present in the buffer cache, thereby saving input/output read operations.
This value is also referred to as session logical reads and is the total of Current Reads plus Consistent Reads.
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