presto-main/src/main/java/io/prestosql/operator/aggregation/multimapagg/GroupedMultimapAggregationState.java/GroupedMultimapAggregationState/add
class GroupedMultimapAggregationState:    @Override
    public final void add(Block keyBlock, Block valueBlock, int position)
    {
        prepareAdd();
        appendAtChannel(VALUE_CHANNEL, valueBlock, position);
        appendAtChannel(KEY_CHANNEL, keyBlock, position);
    }