>>>>>>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 ;-))

So, to be completely clear on the topic of indexes - say, for the First and Last names, should the index be

FirstName, LastName then?

FirstName has less choices than LastName.

The original document, I think, proposed LastName, FirstName index.