A set of objects which are representing the same thing.
A Group may contain different kind of things which are connected by their purpose;
For example currently a group may contain the following objects:
- Operator(s) - which are doing the actual work;
there might be more than one, since an optimization may replace an operator with a new one
- Signature - to enable inter-plan look up of the same data
- OperatorStats - collected runtime information