# Functions
ExecSubPlan used to execute all the children plan.
NewAggregateOperator creates new AggregateOperator.
NewLimitOperator creates the new limit operator.
NewOrderByOperator creates new orderby operator.
# Structs
AggregateOperator represents aggregate operator.
LimitOperator represents limit operator.
OrderByOperator represents order by operator.
# Interfaces
Operator interface.