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Oracle® Database 10g Insider Solutions by Richard Stroupe, John Kanagaraj, Arun Kumar R.

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Effective Use of Histograms

Essentially, a histogram is information about the values in a selected column. Histograms are used in Oracle to describe the skew in the distribution of data in columns that hold non-unique, repeating values. In other words, when a few distinct values in that column form a sizeable portion of the row count, and this count is not uniformly distributed, then histograms can describe this distribution mathematically. Thus, when histograms are created on key columns like those that contain a set of repeating values, such as the state ID in a national cell phone company's customer table, they can quantify the spread of distinct values. Histograms exert a great influence on the Optimizer and are essentially “buckets” that ...

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