Figure 3-5. A hashing structure can be used to quickly locate (search for) a specific object. The memory chains shown in option A on the left are protected by a single latch, whereas with option B, the chains are protected by three latches.

Essentially, a hashing structure is composed of a bunch of chains. When looking for an object (think block buffer or cursor), the searcher is directed to one of the chains. Then the searcher sequentially searches the chain, hoping to find whatever it is seeking. With Oracle's hashing implementation, if the object is not found in the specified chain, then the object is not in the cache.

Option A in Figure 3-5 shows the entire hashing structure is protected-its access is controlled-by a single latch. While this will ensure serialization, with even limited concurrency, it will also ensure significant latch contention. As option B shows, a hashing structure can naturally be divided into group of chains, with each group being protected by its own latch. Each latch in option B in Figure 3-5 will be responsible for serialized access to a specified range of chains. In this example, the number of chains will be either three or four.