Algorithm for the comparer functions in Table.Group, List.Min, List.Max

Hey Mike,

That is an interesting topic. This week I finished an article on using Table.Group with Custom Comparer functions, which can you read here: https://gorilla.bi/power-query/table-group-custom-comparers/

For a Comparer function to be valid, it needs to be symmetric. So defining a comparer function, should result in 1, 0 or -1 as a result. What I mean with symmetric is that if row 1 is evaluated first and row 2 after then the resulting order should be identical to when row 2 is evaluated first and row 1 second.

Now, I don't think the result of evaluating the comparer logic is used independently for each row in the dataset. How I see it, is that the function is applied repeatedly to all the rows, to form groups. So the logic tries row 1 and 2, and says row 2 comes first. It then applies the logic to row 3, and compares it to row 1 and 2, and positions row 3. Then it performs the logic for row 4, and looks where in the order row 4 belongs. 

Also, whereas using GroupKind.Global requires a symmetric comparer function, you can 'get away' with a simplified comparer function when using GroupKind.Local. 

Have a look at my article and I'd be happy to hear your thoughts. You can also see another example illustrated here.

Regards,

Rick