Understanding DAX query plans is a long process. This paper explains how to capture and read a DAX query plan, helping you to find possible optimizations of your DAX queries.
Values such as inventory and balance account, usually calculated from a snapshot table, require the use of semi-additive measures. In Multidimensional you have specific aggregation types, like LastChild and LastNonEmpty. In PowerPivot and Tabular you use DAX, which is flexible enough to implement any calculation, as described in this article.
In a Power Pivot or Tabular model that has inactive relationships, you can use the USERELATIONSHIP function to apply an inactive relationship to a particular DAX expression. Its usage is simple in a measure, but you might consider alternative syntax in a calculated columns, as it is explained in this article.
In the traditional star schema design of a Data Mart, you replace a missing, unknown or wrong date in the fact table with a dummy value in the Date dimension table. In Tabular, handling a Date table requires an existing date and you cannot use a NULL date in a Date table. This article describes how to apply the NULL replacement in the fact table using views, without altering the relational structure of the Data Mart.