柱状图和列统计信息的关系

Histograms是Column Statistics的一部分

有关柱状图更详细的信息可以参考性能优化向导中对应的章节：http://docs.oracle.com/cd/E11882_01/server.112/e16638/stats.htm#PFGRF94724

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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 METHOD_OPT to FOR ALL COLUMNS SIZE 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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