Histograms: An Overview

NOTE: This article was originally created for version 7.3. Conceptually the ideas presented are the same as in later versions but implementation details may have changed or have been enhanced. Histograms ========== For uniformly distributed data, the cost-based approach makes fairly accurate guesses at the cost of executing a particular statement. However, when the data is not uniformly distributed, the optimizer cannot accurately estimate the selectivity of a query. Beginning in release 7.3, for columns which do not have uniform data distribution, Oracle will allow you to store histograms describing the data distribution of a particular column. When to Use Histograms ---------------------- Histograms are stored in the dictionary and computed by using the DBMS_STATS command on a particular column. Therefore, there is a maintenance and space cost for using histograms. You should only compute histograms for columns which you know have highly-skewed data distribution. When to Not Use Histograms -------------------------- Also, be aware that histograms, as well as all optimizer statistics, are static. If the data distribution of a column changes frequently, it is necessary to recompute the histogram for a given column. Histograms are not useful for columns with the following characteristics: