Database Oracle How to measure Index Selectivity Tutorial

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Tutorial: Database Oracle How to measure Index Selectivity Tutorial

B*TREE Indexes improve the performance of queries that select a small percentage of rows from a table. As a general guideline, we should create indexes on tables that are often queried for less than 15% of the table's rows. This value may be higher in sit

Tutorial Details:

How to measure Index Selectivity

Index Selectivity

B*TREE Indexes improve the performance of queries that select a small percentage of rows from a table. As a general guideline, we should create indexes on tables that are often queried for less than 15% of the table's rows. This value may be higher in situations where all data can be retrieved from an index, or where the indexed columns can be used for joining to other tables.

The ratio of the number of distinct values in the indexed column / columns to the number of records in the table represents the selectivity of an index. The ideal selectivity is 1 . Such a selectivity can be reached only by unique indexes on NOT NULL columns.

Example with good Selectivity

A table having 100'000 records and one of its indexed column has 88000 distinct values, then the selectivity of this index is 88'000 / 10'0000 = 0.88.

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