Obscure MDX Month: Optimising The Performance Of Total-To-Date Calculations In SSAS Multidimensional

Reposted from Chris Webb's blog with the author's permission.

Here’s a SSAS Multidimensional MDX tip that I picked up at the PASS Summit back in 2008 at Mosha’s excellent “MDX Deep Dive” precon (incidentally the slides and supporting material are still available here, although a lot of the material is out of date). It’s regarding total-to-date calculations, ie calculations where you are doing a running total from the very first date you have data for up to the current date. The standard way of writing these calculations is something like this:

WITH
MEMBER MEASURES.[TTD Sales] AS
SUM(
NULL:[Date].[Calendar].CURRENTMEMBER,
[Measures].[Internet Sales Amount])

SELECT
[Customer].[Country].[Country].MEMBERS 
ON 0,
NON EMPTY
[Date].[Calendar].[Date].MEMBERS
*
[Product].[Product].[Product].MEMBERS
ON 1
FROM
[Adventure Works]
WHERE(MEASURES.[TTD Sales])

This query runs in around 19.2 seconds on my laptop on a cold cache. However if you rewrite it like this:

WITH
MEMBER MEASURES.[PTTD SALES] AS
SUM(
NULL:[Date].[Calendar].CURRENTMEMBER.PARENT.PREVMEMBER,
[Measures].[Internet Sales Amount])

MEMBER MEASURES.[TTD Sales] AS
MEASURES.[PTTD SALES]
+
SUM(
[Date].[Calendar].CURRENTMEMBER.FIRSTSIBLING:
[Date].[Calendar].CURRENTMEMBER,
[Measures].[Internet Sales Amount])

SELECT
[Customer].[Country].[Country].MEMBERS 
ON 0,
NON EMPTY
[Date].[Calendar].[Date].MEMBERS
*
[Product].[Product].[Product].MEMBERS
ON 1
FROM
[Adventure Works]
WHERE(MEASURES.[TTD Sales])

…it runs slightly faster: around 16.1 seconds on a cold cache on my laptop. Of course this is a very big query, and on most normal queries the difference in performance would be much less significant, but it could still be useful. In fact it’s very similar to the kind of tricks people used to optimise the performance of YTD calculations back in the days of SSAS 2000 – the subject of my second-ever blog post from December 2004! The idea here is that instead of summing up a large set of dates, the calculation sums up all the dates in the current month and then all the months from the beginning of time up to and including the previous full month. For YTD and most other something-to-date calculations trick like this are no longer needed, and indeed are counter-productive and will make your calculations slower. However it seems that for total-to-date calculations they can still help performance.

Chris has been working with Microsoft BI tools since he started using beta 3 of OLAP Services back in the late 90s. Since then he has worked with Analysis Services in a number of roles (including three years spent with Microsoft Consulting Services) and he is now an independent consultant specialising in complex MDX, Analysis Services cube design and Analysis Services query performance problems. His company website can be found at http://www.crossjoin.co.uk and his blog can be found at http://cwebbbi.wordpress.com/ .