Clusters may not be appropriate for tables that regularly require full table scans, in
which a query requires the Oracle database to iterate through all the rows of the
table. Because you access a cluster table through the cluster index, which then points
to a data block, full table scans on clustered tables can actually require more I/O
operations, lowering overall performance.
100 | Chapter 4: Oracle Data Structures
Hash Clusters
A hash cluster is like a cluster with one significant difference that makes it even
faster. Each request for data in a clustered table involves at least two I/O operations,
one for the cluster index and one for the data. A hash cluster stores related data rows
together, but groups the rows according to a hash value for the cluster key. The hash
value is calculated with a hash function, which means that each retrieval operation
starts with a calculation of the hash value and then goes directly to the data block
that contains the relevant rows.
By eliminating the need to go to a cluster index, a hash clustered table can be even
faster for retrieving data than a clustered table.
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