SSAS Multidimensional: Are Your Indexes Processed?

If you are using SSAS Multidimensional and you use Process Update to process your dimensions, here’s something for you to try: run a Process Default on your cube. Does it finish in a few seconds? Then you’re ok. If it doesn’t, and it takes minutes or even longer then read on – you might have a problem that’s causing slow query performance.

One of the most common sources of query performance problems I see with my SSAS Multidimensional customers is unprocessed aggregations and indexes. If you run a Process Update on a dimension it may result in indexes and aggregations being dropped from partitions in your cubes; for more details on this, and why it happens, see this post:

https://blog.crossjoin.co.uk/2010/05/12/what-happens-when-you-do-a-process-update-on-a-dimension/

This classic post by Darren Gosbell explains how you can check if you have unprocessed aggregations on a partition:

http://geekswithblogs.net/darrengosbell/archive/2008/12/02/ssas-are-my-aggregations-processed.aspx

However, unprocessed indexes can also be a problem for query performance too. You can tell if the indexes on a partition are built by using the discover_partition_dimension_stat DMV. Here’s an example of how to use it for a partition in the Adventure Works database:

SELECT 
DIMENSION_NAME, ATTRIBUTE_NAME, ATTRIBUTE_INDEXED, 
ATTRIBUTE_COUNT_MIN, ATTRIBUTE_COUNT_MAX 
FROM SystemRestrictSchema($system.discover_partition_dimension_stat
        ,DATABASE_NAME = 'Adventure Works DW 2008'
        ,CUBE_NAME = 'Adventure Works'
        ,MEASURE_GROUP_NAME = 'Internet Sales'
        ,PARTITION_NAME = 'Internet_Sales_2003')

 

[For some background on running SSAS DMV queries, see here]

Here’s what the above query returns, a list of dimensions and attributes that are related to the partition:

image

If the ATTRIBUTE_INDEXED column shows false then indexes are not built for the attribute on the dimension. In this example no indexes are built at all on the partition; if I do a Process Index or Process Default on this partition, here’s what the DMV returns:

image

Now you can see the ATTRIBUTE_INDEXED property is set to true for most attributes. Note that there is an (All) attribute that is never indexed, and if you have set the AttributeHierarchyEnabled property to false or the AttributeHierarchyOptimizedState property to NotOptimized on an attribute, it will not have indexes built for it either (this is typically done to improve processing performance – see here for a few more details).

In a real-world cube it is likely that only a few indexes will be dropped on partitions as a result of a Process Update on a dimension, and even then this will depend on whether any changes take place in the dimension’s data, so you will have to look down the list of attributes returned by this DMV very carefully to see if ATTRIBUTE_INDEXED returns false when it should be returning true.

The solution to this problem, as several of the posts I’ve linked to above suggest, is to always run a Process Default on your cube as the last step in your processing schedule. A Process Default will process any object that is in an unprocessed state, so it will automatically rebuild any aggregations or indexes that are dropped as a result of Process Updates on dimensions.

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