APPROX_COUNT_DISTINCT( expression ) evaluates an expression for each row in a group, and returns the approximate number of unique non-null values in a group. This function is designed to provide ...

SQL COUNT DISTINCT has been slow. It is, in essence, a grouping operation, which keeps traversed grouping field values for later comparisons. When the result set is too large, the data needs to be ...

Yet another excellent paper on optimising distinct count measures from the SQLCat team: Actually I’m beginning to wonder whether I should be linking to the SQLCat team site – in the sam… ...