Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Determining when to rebuild indexes

Oracle indexes perform two basic operations as the index expands to hold more keys. Oracle's version of b-tree indexing uses an algorithm where each index node may contain many index keys. As new key values are added to the index, Oracle must manage the configuration of each index node. Oracle index nodes are managed with two operations; splitting and spawning.

Splitting-- This is the term used to describe what happens when an index node is filled with keys and a new index node is created at the same level as the full node. Splitting widens the b-tree horizontally.

Spawning-- This is the term used to describe the process of adding a new level to an index. As a new index is populated, it begin life as a single level index. As keys are added, a spawn take place and the first level node re-configures itself to have pointers to lower-level nodes. It is important to understand that spawning takes place at specific points within the index, and not for the entire index. For example, a three level index may have a node that experiences heavy insert activity. This node may spawn a fourth level without all of the other level three nodes spawning new levels.

The index_stats view contains information about the internal structure of the b-tree index that can be useful when determining whether or not to rebuild the index. The following columns of index_stats are especially useful:

height - This columns refers to the maximum number of levels encountered within the index. An index may have 90% of the nodes at 3 levels, but excessive splitting and spawning in one area of the index may have caused some nodes to have move then 3 levels. Whenever the value of height is more than three, you may benefit from dropping and re-creating the index. Oracle indexing will not spawn a fourth level on a clean rebuild until more than ten million kets have been added to the index.

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This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm