DAX Difference between naked SUM and sum wrapped in CALCULATE

Question

What is the difference between naked SUM and sum wrapped in CALCULATE?

Measure1 = SUM( tab[col1]) )
Measure2 = CALCULATE ( SUM( tab         


        

Answer 1

When CALCULATE is used in a row context, transition will occur where each row becomes a filter and the entire table is scanned for each row, using all the columns of the row as a filter. This will have an impact on performance and it could also cause unexpected results if your table has duplicates.

It's possible the two versions you have provided will return the same value. However, if they don't it's probably because of duplicate rows in your table.

You might find this article very useful.