1.explode
hive wiki对于expolde的解释如下：
explode() takes in an array (or a map) as an input and outputs the elements of the array (map) as separate rows. UDTFs can be used in the SELECT expression list and as a part of LATERAL VIEW.
As an example of using explode() in the SELECT expression list, consider a table named myTable that has a single column (myCol) and two rows:
Then running the query:
SELECT explode(myCol) AS myNewCol FROM myTable;
will produce:
2、Lateral View
Lateral View是Hive中提供给UDTF的conjunction，它可以解决UDTF不能添加额外的select列的问题。
Why we need Lateral View？
当我们想对hive表中某一列进行split之后，想对其转换成1 to N的模式，即一行转多列。
hive不允许我们在UDTF函数之外，再添加其它select语句。
如下，我们想将登录某个游戏的用户id放在一个字段user_ids里，对每一行数据用UDTF后输出多行。
select game_id, explode(split(user_ids,'\\[\\[\\[')) as user_id from login_game_log where dt='2014-05-15'
FAILED: Error in semantic analysis: UDTF's are not supported outside the SELECT clause, nor nested in expressions。具体的操作 参考：https://blog.csdn.net/oopsoom/article/details/26001307
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