presto-main/src/main/java/io/prestosql/operator/aggregation/KeyValuePairs.java/KeyValuePairs/add
class KeyValuePairs:    public void add(Block key, Block value, int keyPosition, int valuePosition)
    {
        if (!keyExists(key, keyPosition)) {
            addKey(key, keyPosition);
            if (value.isNull(valuePosition)) {
                valueBlockBuilder.appendNull();
            }
            else {
                valueType.appendTo(value, valuePosition, valueBlockBuilder);
            }
        }
    }