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.
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.
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