Each hash bucket has an associated hash chain. When a searched-for object is hashed to a bucket, the bucket's chain is sequentially searched looking for the object. If the object is not found in the hash chain, we know the object is not anywhere in the entire hashing structure. If the hash chain is short, the sequential search occurs extremely fast. Even better is a zero chain length if the object is not in the cache.

Oracle's CBC structure, as this is called, is a complex memory structure, and Oracle must maintain serialization control. So it employs one of its serialization structures: a latch or a mutex. As of Oracle Database 11g, Oracle is still using the latching scheme.

Figure 6-9 is a small example of an Oracle hashing structure consisting of two CBC latches (CS 800 and CS 810), six CBCs (CBC 00 through CBC05), and eleven buffer headers (BH nnnn). This is a very small example, but it does show all the key pieces with their proper associations. It is common for Oracle CBCs to have a zero length. Oracle does this to help minimize collisions. As I'll describe next, Oracle goes to great lengths to minimize collisions, and only a very unfortunate situation would lead to a buffer chain more than six buffer headers long.