presto-main/src/main/java/io/prestosql/operator/aggregation/multimapagg/MultimapAggregationStateFactory.java/MultimapAggregationStateFactory/createSingleState
class MultimapAggregationStateFactory:    @Override
    public MultimapAggregationState createSingleState()
    {
        return new SingleMultimapAggregationState(keyType, valueType);
    }