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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Understanding Oracle indexes

* BLEVEL--This is the number of levels that the index has spawned. Even for very large indexes, there should never be more than four levels. Each BLEVEL represents an additional I/O that must be performed against the index tree.

* LEAF_BLOCKS--This is a reference to the total number of leaf blocks.

* DISTINCT_KEYS--This is a reference to the cardinality of the index. If this value is less than 10, you may want to consider redefining the index as a bitmapped index.

* AVG_DATA_BLOCKS_PER_KEY--This is a measure of the size of the index and the cardinality of the index. A low cardinality index (i.e., sex or region) will have high values, as will very large indexes.

* CLUSTERING_FACTOR--This is the most important measure in this report, since it measures how balanced the index is, relative to the table. If the clustering factor is near the number of blocks in the table, then the table is said to be clustered within the index. This is good for data retrieval with the index, since there will be less physical I/O. If the clustering factor approaches the number of rows in the table, then the index is said to be random. That is, the index keys are not in the same physical order as the rows in the table. Of course, only one index on a table will have a high clustering factor, since the rows can only be physically ordered to match one index key value.

* AVG_LEAF_BLOCKS_PER_KEY--This is always one, with the exception of non unique indexes.
These statistics give Oracle’s cost-based optimizer clues about which indexes are best suited to servicing each query.

This is an excerpt from "High Performance Data Warehousing", copyright 1997. 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:



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