Is there a performance gain using + rather than Aggregate(Union

问题

I was looking at this post in MSDN: http://msdn.microsoft.com/en-us/library/ms145493.aspx

It states that + and Union are equivalent ... not exactly in the following as Aggregate needs to be applied if I choose to use Union.

It's more readable but is there any performance gain in choosing + over Aggregate(Union ?

WITH 
  MEMBER [Employee].[Employee].[blah] AS 
      [Employee].[Employee].[Amy E. Alberts]
    + 
      [Employee].[Employee].[Garrett R. Vargas] 
  MEMBER [Employee].[Employee].[blahblah] AS 
    Aggregate
    (
      Union
      (
        {[Employee].[Employee].[Amy E. Alberts]}
       ,{[Employee].[Employee].[Garrett R. Vargas]}
      )
    ) 
SELECT 
  [Measures].[Reseller Sales Amount] ON 0
 ,{
    [Employee].[Employee].[Amy E. Alberts]
   ,[Employee].[Employee].[Garrett R. Vargas]
   ,[Employee].[Employee].[blah]
   ,[Employee].[Employee].[blahblah]
  } ON 1
FROM [Adventure Works]
WHERE 
  [Date].[Calendar].[Month].&[2006]&[8];

回答1:

The issue is that + is used for several purposes in MDX: as numerical addition, for string concatenation, and as union operator. And as the a member definition expects an expression that delivers a value and you use the operator between two members, and the value of the current measure is numerical, AS uses numerical addition.

If you enclose the members in braces:

  MEMBER [Employee].[Employee].[blah] AS 
      {[Employee].[Employee].[Amy E. Alberts]}
    + 
      {[Employee].[Employee].[Garrett R. Vargas] }

you get an error that AS expects string or numeric expressions for the + operator.

And there is probably a small performance gain, but the result may be different if the current measure is not aggregated by sum, which would be equivalent to adding both member values in this case.

I personally do not like this operator overloading of +, * etc. for different purposes. + and * should be restricted to numbers, be it integers, floats, or decimals. I also would also prefer another operator for string concatenation than + in Java, JavaScript, and .net, as well as Microsoft SQL and MDX. Standard SQL uses ||, Perl uses ., so there are alternatives that clearly avoid ambiguities and you clearly know what to expect when you write '2' || 1 vs. '2' + 1 vs. vs. 2 + '1' vs. 2 + 1.

来源：https://stackoverflow.com/questions/26886382/is-there-a-performance-gain-using-rather-than-aggregateunion

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