Using COMBINEVALUES to optimize DirectQuery performance
The DAX language manages data models in Power BI, Power Pivot, and Analysis Services Tabular. To simplify the reference to data models where DAX can be used, we call them Tabular models. A relationship in a Tabular model connects two tables only using one column as an identifier. While this seems like a limitation, it reflects the behavior of the internal VertiPaq engine, which is an in-memory columnar database.