Window functions (INDEX, OFFSET, WINDOW, RANK, and ROWNUMBER) are the latest additions to the DAX language; they also introduce a new concept named apply semantics. They are extremely powerful in simplifying writing DAX code that needs to work on sorted tables while sometimes (but not always) increasing performance for several calculations, like running totals, comparing one row with the previous one, etc.
In this session, we introduce window functions and apply semantics through several examples to understand their capabilities and performance.