how to get the key in Map with SQL

[??????????]

Hi all,


I have a table looks like:
+---------+---------+
|A|B|
|0|[Map(1->a),1]|
|0|[Map(1->b),2]|


 I want to pickup the key and value in Map.


My code looks like df.select($"B._2".alias("X"),$"B._1.key".alias("Y")).show


The output is 
|X|Y|
|1|null|
|2|null|



Would you like tell me how to get the key in Map please?


My spark is 2.1.0


Thanks
Fei Shao