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Oracle Tips by Burleson

Index-only Tables

• The ORGANIZATION INDEX specification indicates that the store_table is an index-only table. The row data resides in an index defined on the store_id column that is the store_table’s primary key.
   

• The OVERFLOW TABLESPACE specification indicates that rows that exceed 20% of the table’s blocksize will be placed in the store_table_ovflw tablespace.

Single Table Hash Clusters

Hashing is another way of storing table data to improve the performance of data retrieval. The administrator first creates the hash cluster storage object and then creates the tables specifying the cluster as the storage mechanism.  Oracle physically stores the rows of a table in a hash cluster and retrieves them according to the results of a hash function (much like the IMS hierarchical database).

The hash function uses a key value to calculate an address, which corresponds to a specific data block in the cluster. A cluster key column value is specified during the creation of the cluster. The key values for a hash cluster can be loosely compared to index key values as they can consist of a single key column or composite key column (multiple table columns make up the key). 

To insert or retrieve a row from the hash cluster, Oracle uses the hash function to calculate the address of the data block the row is stored in. To find a row by using an index, Oracle must traverse the index to find the key value/ROWID combination and then incur additional I/O to read the row from the table.  Because the hash function calculates the block address of the requested row, no index searches are required.
 

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