直方图是许多商用数据库系统中最常用的一种估算查询结果大小的方法。

Histogram is the common method for size estimation of query result in many commercial database systems.

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查询是数据库系统中最基本也是最常用的一种操作，因此查询是否具有较快的执行速度，已成为数据库用户和设计者极其关心的问题。

Query is one of the basic and commonly using operations in DBMS. So whether the query has the fast execution speed has become a core problem for the users and designers of DBMS.

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