This expression
(@Value(part_count)==0 && @Value(OBJECTID)==1)?1:
(@Value(part_count)==0 && @Value(OBJECTID)!=1)?@Value(feature[-1].Dag_GB)+1:@Value(feature[-1].Dag_GB)
It is a working partitioniser.
It works by first using a counter (counter4 in picture) in group by mode on wathever you want to partitionise. Data must be sorted first (mandatory).
Then it runs the expression on the resulting counter attribute.
This is way faster then aggregating(listbuilding) by group, counting and then deaggregating(exploding).
It is faster because it does not block. It streams nicely.
But the wheels stays red! (attributecreator4)
This is annoying.
A known bug?
(using 2018 B18301)