Index-organized table (IOT) vs. Cluster table Don Burleson

I'm often asked "when should I use an IOT vs. a single-table cluster?".  Both IOT and cluster tables are used to reduce I/O (consistent gets and physical reads) by making the SQL more efficient.  The SQL become more efficient on an IOT and a cluster table because less access is required to gat all of the data to satisfy the request.  Another approach is Oracle Row Re-sequencing, to perform this manually.

Also see: Super App to use Oracle IOT's

There are fundamental differences in the table internal structure of an index-organized table (IOT) and a single-table cluster table, and you need ti understand how your table is being accessed:

• IOT - Index-organized tables - These structures remove the primary key from the table and keep them solely in the index, saving disk space.  Primarily used with high-updates tables, IOT structures reduce table fragmentation.  Dr. Tim Hall, author of the book "Oracle Job Scheduling" has a great discussion of IOT benefits:
   
  • Accessing data via the primary key is quicker as the key and the data reside in the same structure. There is no need to read an index then read the table data in a separate structure.
     
  • Lack of duplication of the key columns in an index and table mean the total storage requirements are reduced.
     
• Single table clusters - A single-table index cluster table is a method whereby Oracle guarantees row sequence where clustering_factor in dba_indexes always approximates blocks in dba_tables.  Hashing is used to enforce key-order, and can greatly reduce I/O if your SQL commonly does multi-block index range scans.  See this link for more information on Oracle cluster tables.  However, single table clusters require maintenance when the overflow area is used.

Alternatives to IOT's

Oracle offers several other alternatives to index organized tables:

• Click here to view a talking video example of the benefits of row re-sequencing in Oracle:
   
• Learn how Oracle hash clustering can re-sequence table rows for faster SQL execution.
   
• Leverage Oracle Sorted Hash Clusters, a great automatic method for enforcing physical row sequence at the data block level.

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