Dany Hoter needs a distinct count:
Calculating distinct counts on massive distributed datasets is not trivial.
Kusto (aka Azure Data Explorer/KQL database in Fabric) dcount and dcountif functions use a special algorithm to return an estimate of distinct counts
The new functions count_distinct and count_distinctif were recently added to calculate exact distinct counts. These two functions are much more expensive than the original ones.
Read on for more details on how this all works.