presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/Visitor/planGlobalAggregation
class Visitor:        private PhysicalOperation planGlobalAggregation(AggregationNode node, PhysicalOperation source, LocalExecutionPlanContext context)
        {
            ImmutableMap.Builder<Symbol, Integer> outputMappings = ImmutableMap.builder();
            AggregationOperatorFactory operatorFactory = createAggregationOperatorFactory(
                    node.getId(),
                    node.getAggregations(),
                    node.getStep(),
                    0,
                    outputMappings,
                    source,
                    context,
                    node.getStep().isOutputPartial());
            return new PhysicalOperation(operatorFactory, outputMappings.build(), context, source);
        }