Matt Allington continues a discussion on the FILTER() function in DAX:
The new formula follows the rule “don’t filter a table if you can filter a column”. But in this case the column and the table have the same cardinality, so there is little benefit there. Also, the new formula requires a second CALCULATE() and SUM() inside the FILTER() function. This is required because the column Customers[YearlyIncome] is no longer in the same table that FILTER() is iterating. The FILTER() function is iterating a virtual, single column table that contains all customer keys in the customer table. The column Customers[YearlyIncome] doesn’t exist in this virtual table, it exists in the Customers table, so you must wrap the column in an aggregation function, SUM() in this case. Further, as the FILTER() function iterates in a row context through the virtual table, the virtual relationship does not filter the connected tables UNLESS you specifically tell the formula to do so. Technically, to make the filter propagate from the new virtual table created by ALL(Customers[CustomerKey]), we need to convert the row context into an equivalent filter context via context transition. Context transition is triggered by the inner CALCULATE() inside the FILTER() function in this case.
Read on for several tips for efficient filtering.