public AggregateExpression toAggregateExpression() {
        return new AggregateExpression(this, AggregateMode.Complete);
    }
Wraps this [[AggregateFunction]] in an [[AggregateExpression]] and set isDistinct
field of the [[AggregateExpression]] to the given value because
[[AggregateExpression]] is the container of an [[AggregateFunction]], aggregation mode,
and the flag indicating if this aggregation is distinct aggregation or not.
An [[AggregateFunction]] should not be used without being wrapped in
an [[AggregateExpression]].
indexr-query-opt/src/main/java/io/indexr/query/expr/agg/AggregateFunction.java