Clustered tables also reduce the amount of space needed to store the columns the two tables
have in common, also known as a cluster key value. The cluster key value is also stored in a
cluster index. The cluster index operates much like a traditional index in that it will improve
queries against the clustered tables when accessed by the cluster key value. In our example with
orders and line items, the order number is only stored once, instead of repeating for each lineitem
detail row.
The advantages to clustering a table are reduced if frequent insert, update, and delete
operations occur on the table relative to the number of select statements against the table. In
addition, frequent queries against individual tables in the cluster may also reduce the benefits
of clustering the tables in the first place.
Hash Clusters
A special type of clustered table, a hash cluster, operates much like a regular clustered table,
except that instead of using a cluster index, a hash cluster uses a hashing function to store and
retrieve rows in a table. The total estimated amount of space needed for the table is allocated
when the table is created, given the number of hash keys specified during the creation of the
cluster. In our order-entry example, let??™s assume that our Oracle database needs to mirror the
legacy data-entry system, which reuses order numbers on a periodic basis. Also, the order number
is always a six-digit number.
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