In my last post I promised to cover the Segment iterator in more detail, so here we go.
Segment
The Segment iterator partitions rows into groups as they flow through a query plan, checking whether the current row belongs in the same group as the previous row. For this to work, the incoming rows must be presented in an order which guarantees that all members of a group are received sequentially.
Segment has a “Group By” argument to specify how it should partition its input. The iterator adds an additional column to the rows that flow through it. This new column is used to communicate with its parent iterator, and is named something like [Segment1003].
The new column is visible in the graphical query plan properties window, or by hovering your mouse cursor over the Segment operator. The attribute name in both cases is Segment Column).