How to use lateral view explode() function in Hive

In Hive, we can create tables which has the MAP structure inside, like:

create table test (
    item MAP<STRING, STRING>
);

and sometimes we want to iterate all the items inside the MAP as key-value pairs.
Hive offered such function called explode():

explode() takes in an array as an input and outputs the elements of the array as separate rows. UDTF's can be used in the SELECT expression list and as a part of LATERAL VIEW.
An example use of explode() in the SELECT expression list is as follows:
Consider a table named myTable that has a single column (myCol) and two rows:

Array<int> myCol
[1,2,3]
[4,5,6]

Then running the query:
SELECT explode(myCol) AS myNewCol FROM myTable;
Will produce:

(int) myNewCol
1
2
3
4
5
6

the above is extracted from the official guide:

and here I am going to take a tour through how to explode the MAP structure and also how to explode multiple MAP structure 

taking `test` table as an example:

hive>select * from test;

{"123":"abc"}

{"234":"bcd"}

now if we do :

hive>select key, value from test 

    lateral view explode(item) dummy_table as key, value;

123    abc

234    bcd

as we can see, explode will expand the MAP into multiple rows. and of course we can use key, value in any clause like 'group by' or 'sort by' etc.

and how about if we have multiple items which are all MAP structure, like :

create table test2 (

    item1 MAP<STRING, STRING>,

    item2 MAP<STRING, STRING>

)

hive>select * from test2;

{"123":"abc","234":"bcd"}  {"123":"aaa","234":"bbb"}

now if we do the same query again:

hive>select key1, value1, key2, value2 from test2 

                  lateral view explode(item1) dummy1 as key1, value1

                  lateral view explode(item2) dummy2 as key2, value2;

123 abc 123 aaa

123 abc 234 bbb

234 bcd 123 aaa

234 bcd 234 bbb

we see that Hive won't show just two lines, instead, it will try all combinations.

so now we have a problem, how about I wanna do sum up on item1["123"] ?

if the value of key "123" is not the alphabet but number instead, I should be able to sum up the value based on the key, right ?

but now, as we showed above, Hive will do combination, so the value will be duplicated!

Here is my solution, and simple:

hive>select key1, value1, key2, value2 from test2 

                  lateral view explode(item1) dummy1 as key1, value1

                  lateral view explode(item2) dummy2 as key2, value2

          where key1 = key2;

123 abc 123 aaa

234 bcd 234 bbb

so now if we do sum up like :

hive>select key1,SUM(value1), SUM(value2) from test2

                  lateral view explode(item1) dummy1 as key1, value1

                  lateral view explode(item2) dummy2 as key2, value2

          where key1 = key2;

we will get the correct sum up value of every key.