SQL Server自动更新统计信息的基本算法
最初接触SQL Server的时候认为SQLServer数据更改的同时就会相应的更新统计信息,其实SQL Server不是这样做的.基于性能考虑,SQL Server使用下面的算法更新统计信息
自动更新统计信息的基本算法是:· 如果表格是在 tempdb 数据库表的基数是小于 6,自动更新到表的每个六个修改。
· 如果表的基数是大于 6,但小于或等于 500,更新状态每 500 的修改。
· 如果基数大于 500,表为更新统计信息时(500 + 20%的表)发生了更改。
· 表变量为基数的更改不会触发自动更新统计信息。
注意:此严格意义上讲,SQL Server 计算基数为表中的行数。
注意:除了基数,该谓语的选择性也会影响 AutoStats 生成。这意味着该统计信息可能无法更新的 afer 每 500 修改基数是 500。比例因子(值的范围从 1 到 4、 1 和 4 之间)上生成根据选择性,这一因素的更改从算法获得数的乘积 AutoStats 生成所需的修改的实际数目。
在表的窗体中,可以summarised 以上算法:
_________________________________________________________________________________
Table Type | Empty Condition | Threshold WhenEmpty |Threshold When Not Empty
_________________________________________________________________________________
Permanent | = 500 | # of Changes >=500 + (20% of Cardinality)
___________________________________________________________________________
Temporary | = 6 | # of Changes >=500 + (20% of Cardinality)
___________________________________________________________________________
Table
Variables | Change in cardinality does not affectAutoStats generation.
___________________________________________________________________________

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