Microsoft has discovered an issue with turning on the "hiding" feature of "tabular functions" to improve query performance. This is the statement from the product team (from the forum) -
"FIM 2010 Update Rollup 2 (build 4.0.3606.2) contains a feature that is intended to improve Query performance in the case of certain complex queries. This “tabular functions” feature is turned off by default. The product team wanted to get experience among a controlled group of customers before announcing the feature for general use. However, we inadvertently included information about this feature in the original KB for Update Rollup 2 and have since removed it. However, we understand that information about how to turn on the feature is circulating among the FIM community. Once the feature is turned on it cannot be turned off.
The product team has discovered an issue in this feature that could return incorrect query results when the query includes at least two statements and the same attribute is referenced in the statements. For example:
/Person[(AccountName=’A’ and JobTitle=’Developer’) or (AccountName’=’B’ and JobTitle=’Accountant’)]
Since it is likely that typical installations will use this query pattern we strongly advise customers NOT to turn on the Set Partition feature. We will update the community as we understand the issue more completely and have developed a recommendation for addressing it."
As you can see, it is strongly recommended NOT to turn on the Set Partition feature - and that it cannot be disabled once activated.
You can and should follow the thread in the FIM Forum (including the excerpt) here.