Dax Formula Optimization - Counting transactions based on two facts

The golden rule of DAX says: "You should NEVER filter a table when you can filter a column." One of the reasons is... speed. You are filtering the full expanded(!!!) table Sales and put it all as a filter. This is one of the worst things you can do in DAX. Instead, you should always filter columns only.

// Assumption is that Store and Calendar
// are dimensions connected to 2 fact
// tables, Sales and Footfall, and the
// connection in 1:* and the filtering
// is one-way.

Transactions =
var __DaysAndStoresWithFootfall =
	CALCULATETABLE(
		SUMMARIZE(
			Footfall,
			Store[StoreID],
			Calendar[Date]
		),
		// No columns from fact tables
		// should ever be exposed to
		// the end user. If you do
		// expose them (very bad),
		// you have to wrap this condition
		// in KEEPFILTERS.
		Footfall[Footfall] > 0
	)
var __result =
	CALCULATE (
		// Why not DISTINCTCOUNT?
		DISTINCTCOUNTNOBLANK ( Sales[Transaction key] ),
		__DaysAndStoresWithFootFall,
		ALL( Stores ),
		ALL( 'Calendar' )
	)
return
	__result