presto-main/src/main/java/io/prestosql/operator/aggregation/multimapagg/SingleMultimapAggregationState.java/SingleMultimapAggregationState/add
class SingleMultimapAggregationState:    @Override
    public void add(Block key, Block value, int position)
    {
        keyType.appendTo(key, position, keyBlockBuilder);
        valueType.appendTo(value, position, valueBlockBuilder);
    }