APPROXIMATEDISTINCTCOUNT Returns the approximate number of rows that contain distinct values in a column. This function can query large amounts of data with potentially better performance than DISTINCTCOUNT, with slight deviation from the exact result.
Syntax
APPROXIMATEDISTINCTCOUNT(columnName)
Parameters
column – The column that contains the values to be counted. This cannot be an expression.
Return Value
The approximate number of distinct values in column.
Reference Data
https://docs.microsoft.com/en-us/dax/dax-function-reference