Assume we group on column 2. Assume that value of col2 can never change back to a previous value (e.g. by sorting it).

I.e. this is illegal:

44,a
12,a
15,a
10,B
12,B
33,a

But this is legal:

44,a
12,a
15,a
10,C
12,C
33,b

In the second case it is possible to use --pipepart to binary search to find end of each record, and thus be able to use -N.

If this is implemented maybe warn user that the value to group by cannot change back to a previous value.

"Warning: --group-by --pipepart requires all the same values are grouped together. Mixed sequences like: foo, bar, foo will not work. If in doubt, sort the input on the column, so the sequence is bar, foo, foo."

"Warning: When using --group-by --pipepart, ensure identical values are consecutively grouped. Mixed sequences (e.g. foo, bar, foo) are unsupported. If in doubt sort the input on the column to ensure a consistent sequence (e.g. bar, foo, foo)."