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An Index can improve the performance of queries that select a small percentage of rows from a table. A general guideline is to create indexes on tables that are queried for less than 2% - 4% of the table's rows, with the following assumptions:
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Rows with the same value for the column on which the query is based are uniformly distributed throughout the data blocks allocated to the table
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Rows in the table are randomly ordered with respect to the column on which the query is based
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Each data block allocated to the table contains at least 10 rows
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The table contains a relatively small number of columns
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Most queries on the table have relatively simple WHERE clauses
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