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。
1. 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。
提示语法分析错误，UDTF不支持函数之外的select 语句，真无语。。。
如果我们想支持怎么办呢？接下来就是Lateral View 登场的时候了。
2. Lateral View explain
2.1 单个Lateral View
Lateral view is used in conjunction with user-defined table generatingfunctions such as explode(). As mentioned in Built-in Table-Generating Functions, a UDTF generates zero or more output rows foreach input row. A lateral view first applies the UDTF to each row of base tableand then joins resulting output rows to the input rows to form a virtual tablehaving the supplied table alias.
解释一下：
Lateral view 其实就是用来和像类似explode这种UDTF函数联用的。lateral view 会将UDTF生成的结果放到一个虚拟表中，然后这个虚拟表会和输入行即每个game_id进行join 来达到连接UDTF外的select字段的目的。
Lateral View Syntax
lateralView: LATERAL VIEW udtf(expression) tableAlias AS columnAlias (',' columnAlias)*
fromClause: FROM baseTable (lateralView)*
可以看出，可以在2个地方用Lateral view： 1. 在udtf前面用 2. 在from baseTable后面用
举个例子： 1. 先创建一个文件，里面2列用\t分割，gameid和userids
hive> create table test_lateral_view_shengli(game_id string,userl_ids string) row format delimited fields terminated by '\t' stored as textfile;
OK
Time taken: 2.451 seconds
hive> load data local inpath '/home/hadoop/test_lateral' into table test_lateral_view_shengli;
Copying data from file:/home/hadoop/test_lateral
Copying file: file:/home/hadoop/test_lateral
Loading data to table dw.test_lateral_view_shengli
OK
Time taken: 6.716 seconds
hive> select * from test_lateral_view_shengli;
OK
game101 15358083654[[[ab33787873[[[zjy18052480603[[[shlg1881826[[[lxqab110
game66 winning1ren[[[13810537508
game101 hu330602003[[[hu330602004[[[hu330602005[[[15967506560
下面使用lateral_view
hive> select game_id, user_id
> from test_lateral_view_shengli lateral view explode(split(userl_ids,'\\[\\[\\[')) snTable as user_id
> ;
Total MapReduce jobs = 1
Launching Job 1 out of 1
Number of reduce tasks is set to 0 since there's no reduce operator
Starting Job = job_201403301416_445839, Tracking URL = http://10.1.9.10:50030/jobdetails.jsp?jobid=job_201403301416_445839
Kill Command = /app/home/hadoop/src/hadoop-0.20.2-cdh3u5/bin/../bin/hadoop job -Dmapred.job.tracker=10.1.9.10:9001 -kill job_201403301416_445839
2014-05-16 17:39:19,108 Stage-1 map = 0%, reduce = 0%
2014-05-16 17:39:28,157 Stage-1 map = 100%, reduce = 0%
2014-05-16 17:39:38,830 Stage-1 map = 100%, reduce = 100%
Ended Job = job_201403301416_445839
OK
game101 hu330602003
game101 hu330602004
game101 hu330602005
game101 15967506560
game101 15358083654
game101 ab33787873
game101 zjy18052480603
game101 shlg1881826
game101 lxqab110
game66 winning1ren
game66 13810537508
2.2 多个Lateral View
From语句后可以跟多个Lateral View。
A FROM clause can have multiple LATERAL VIEW clauses. Subsequent LATERAL VIEWS can reference columns from any of the tables appearing to the left of the LATERAL VIEW.
给定数据：
|
Array<int> col1 |
Array<string> col2 |
|
[1, 2] |
[a", "b", "c"] |
|
[3, 4] |
[d", "e", "f"] |
|
int myCol1 |
string myCol2 |
|
1 |
"a" |
|
1 |
"b" |
|
1 |
"c" |
|
2 |
"a" |
|
2 |
"b" |
|
2 |
"c" |
|
3 |
"d" |
|
3 |
"e" |
|
3 |
"f" |
|
4 |
"d" |
|
4 |
"e" |
|
4 |
"f" |
我们可以这样写：
SELECT myCol1, myCol2 FROM baseTable
LATERAL VIEW explode(col1) myTable1 AS myCol1
LATERAL VIEW explode(col2) myTable2 AS myCol2;
- Outer Lateral View 还有一种情况，如果UDTF转换的Array是空的怎么办呢？ 在Hive0.12里面会支持outer关键字，如果UDTF的结果是空，默认会被忽略输出。 如果加上outer关键字，则会像left outer join 一样，还是会输出select出的列，而UDTF的输出结果是NULL。
hive> select * FROM test_lateral_view_shengli LATERAL VIEW explode(array()) C AS a ;
结果是什么都不输出。
如果加上outer关键字：
SELECT * FROM src LATERAL VIEW OUTER explode(array()) C AS a limit 10;
238 val_238 NULL
86 val_86 NULL
311 val_311 NULL
27 val_27 NULL
165 val_165 NULL
409 val_409 NULL
255 val_255 NULL
278 val_278 NULL
98 val_98 NULL
...
4.总结：
Lateral View通常和UDTF一起出现，为了解决UDTF不允许在select字段的问题。 Multiple Lateral View可以实现类似笛卡尔乘积。 Outer关键字可以把不输出的UDTF的空结果，输出成NULL，防止丢失数据。