>>>>>If I read that right then my supposition was correct ? :
>>>>>"Additional columns should be ordered based on their level of distinctness, that is, from the most distinct to the least distinct."
>>>>
>>>>In other words, from the most selective (e.g. column with many unique values) to the less selective (column with less values). This is how I understand it and this is what I said.
>>>
>>>You said: "If you have many categories and just few sites, then using Category as first column "
>>
>>Right, this is how I understand the above sentence. Are you saying that my understanding is wrong?
>
>Seems so to me. In your example categories is the 'least distinct' but you advocate putting it before sites.....

If you allow me to nitpick: it is less the count of distinct values, but more the number of values excluded averaged across all values, which could be visualized as a kinda reverse distribution function. 100 possible values, 99 filled, but having a count of less than 3 and 199K on the last value is less selective as 15 distinct values with each grouped to a count between 10k and 20k ;-))