You can control the number of possible
hash values for a hash cluster with the HASHKEYS parameter when you create
the cluster.
Because the hash cluster directly points to the location of a row in the table, you
must allocate all the space required for all the possible values in a hash cluster when
you create the cluster.
Hash clusters work best when there is an even distribution of rows among the various
values for the hash key. You may have a situation in which there is already a
unique value for the hash key column, such as a unique ID. In such situations, you
can assign the value for the hash key as the value for the hash function on the unique
value, which eliminates the need to execute the hash function as part of the retrieval
process. In addition, you can specify your own hash function as part of the definition
of a hash cluster.
Oracle Database 10g introduced sorted hash clusters, where data is not only stored
in a cluster based on a hash value, but is also stored in the order in which it was
inserted. This data structure improves performance for applications that access data
in the order in which it was added to the database.
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