Aggregate() vs. A Set in the WHERE caluse

Hi,

I recently wrote some VBA code for Excel, which allowed a user to specify a list of members from a particular dimension and then produce a report which aggregated some measures with the members of a different dimension on the rows. It did this by creating an MDX query with the listed members fed into the aggregate function and stored in a calculated member which was then added to the WHERE clause.

eg

WITH MEMBER [Dimension].[Selected_Members] As Aggregate({[Dimension].[Member1], [Dimension].[Member2],.....})
SELECT [Measures].[Measure1] ON COLUMNS,
NON EMPTY [AnotherDimension].Members ON ROWS
FROM [Cube]
WHERE ([Dimension].[Selected_Members])

We found that as the number of members specified increased, the report slowed down dramatically. So, remembering that AS2005 allows you to specify sets in the WHERE clause of your MDX we decided to change it to something like this:

SELECT [Measures].[Measure1] ON COLUMNS,
NON EMPTY [AnotherDimension].Members ON ROWS
FROM [Cube]
WHERE ({[Dimension].[Member1], [Dimension].[Member2],.....})

We were quite surprised to see a very significant improvement in speed and the same results.

So, it would appear that the end result is exactly the same, but I would like to understand what the difference is and why one is so much faster than the other.

Many thanks,

Stuart

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I am surprised to see performance difference between these two examples. I would think that the execution plans should be exactly the same. Perhaps there is something going on in the cube.

Please note, that Aggregate cannot always be exchanged to set in WHERE clause, the results could become different. But if the set to Aggregate over is as static as in your example, it should usually be a safe rewrite.