latch: cache buffers chains

Waits on the cache buffer chains latch, ie the wait event “latch: cache buffers chains” happen when there is extremely high and concurrent access to the same block in a database. Access to a block is normally a fast operation but if concurrent users access a block fast enough, repeatedly then simple access to the block can become an bottleneck. The most common occurance of cbc (cache buffer chains) latch contention happens when multiple users are running nest loop joins on a table and accessing the table driven into via an index.

https://sites.google.com/site/embtdbo/wait-event-documentation/oracle-latch-cache-buffers-chains

Hot blocks are another common cause of cache buffers chains latch contention. This happens when multiple sessions repeatedly access one or more blocks that are protected by the same child cache buffers chains latch. This is mostly an application issue. In most cases, increasing the number of cache buffers chains latches will do little to improve performance. This is because blocks are hashed to hash buckets and chains based on the block address and the number of hash buckets, and not the number of cache buffers chains latches. If the block address and the number of hash buckets remain the same, chances are those few hot blocks will still be covered by one cache buffers chains latch, unless the number of latches is drastically increased.

http://www.confio.com/logicalread/oracle-hot-blocks-cause-contention-mc01/#.U_2D8PmSwbh

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