Column Statistics and Histograms
When gathering statistics on a table,
DBMS_STATS gathers information about the data distribution of the columns within the table. The most basic information about the data distribution is the maximum value and minimum value of the column. However, this level of statistics may be insufficient for the optimizer's needs if the data within the column is skewed. For skewed data distributions, histograms can also be created as part of the column statistics to describe the data distribution of a given column. Histograms are described in more details in "Viewing Histograms".
Histograms are specified using the
METHOD_OPT argument of the
DBMS_STATS gathering procedures. Oracle recommends setting the
AUTO. With this setting, Oracle Database automatically determines which columns require histograms and the number of buckets (size) of each histogram. You can also manually specify which columns should have histograms and the size of each histogram.
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