The Approximate Distinct Count function, which is composed of the ApproxDCountReduce and ApproxDCountMap functions, can estimate the number of distinct values (cardinality) in a column or combination of columns, scanning the table only once.
This function is recommended when the column or combination of columns has a large cardinality. The function can estimate the number of distinct values much faster than the SQL SELECT DISTINCT command can return the precise number of distinct values.
When the cardinality is small, the SQL SELECT DISTINCT command is recommended.