presto-main/src/main/java/io/prestosql/sql/planner/GroupingOperationRewriter.java/GroupingOperationRewriter/translateFieldToInteger
class GroupingOperationRewriter:    private static int translateFieldToInteger(FieldId fieldId, RelationId requiredOriginRelationId)
    {
        // TODO: this section should be rewritten when support is added for GROUP BY columns to reference an outer scope
        checkState(fieldId.getRelationId().equals(requiredOriginRelationId), "grouping arguments must all come from the same relation");
        return fieldId.getFieldIndex();
    }