This section presents the advantages and disadvantages of hashing to help you decide if it is appropriate for your situation. If you opt to use indexing rather than hashing, consider whether to store a table individually or as part of a cluster. The tables in the hash cluster are primarily static in size so that you can determine the number of rows and amount of space required for the tables in the cluster. If tables in a hash cluster require more space than the initial allocation for the cluster, performance degradation can be substantial because overflow blocks are required. The table is not static and continually growing.
Applications frequently perform full-table scans on the table and the table is sparsely populated.
A full-table scan in this situation takes longer under hashing. You cannot afford to pre-allocate the space that the hash cluster will eventually need. See the Oracle8i Application Developer's Guide - Fundamentals for additional recommendations. In this example, it is a single column. Oracle rounds the number specified to the nearest prime number.