什么是SQL Server中的不精确列？

Question

上nvarchar类型的计算列创建索引提出了以下错误：

Cannot create index or statistics 'MyIndex' on table 'MyTable' because the computed column 'MyColumn' is imprecise and not persisted. Consider removing column from index or statistics key or marking computed column persisted.

是什么不精确列是什么意思？

UPDATE。定义如下：

alter table dbo.MyTable 
    add [MyColumn] as dbo.MyDeterministicClrFunction(MyOtherColumn) 
go 
create index MyIndex on dbo.MyTable(MyColumn) 
go 

UPDATE2。该MyDeterministicClrFunction定义如下：

[SqlFunction(IsDeterministic = true)] 
public static SqlString MyDeterministicClrFunction(SqlString input) 
{ 
    return input; 
} 

Answer 1

Per MSDN, CLR Function columns must be persisted to be indexed:

Any computed column that contains a common language runtime (CLR) expression must be deterministic and marked PERSISTED before the column can be indexed. CLR user-defined type expressions are allowed in computed column definitions. Computed columns whose type is a CLR user-defined type can be indexed as long as the type is comparable. For more information, see CLR User-Defined Types.

坚持列，我怀疑它会工作。

Answer 2

从SQL server documentation：

Any float or real expression is considered imprecise and cannot be a key of an index; a float or real expression can be used in an indexed view but not as a key. This is true also for computed columns. Any function, expression, or user-defined function is considered imprecise if it contains any float or real expressions. This includes logical ones (comparisons).

Answer 3

你试过：

[SqlFunction(IsDeterministic=true, IsPrecise=true)] 

http://msdn.microsoft.com/en-us/library/orm-9780596101404-02-12.aspx

http://msdn.microsoft.com/en-us/library/ms189292.aspx

听起来就像是错误消息误导，因为CLR计算列必须坚持反正（被编入索引）。