我文章提到，加载一个文件的部分列是可行。两列，你只读一列，没问题。
但是，两个文件，f1和f2，f1有42列，f2有43列，同时加载到一个流对象，如何？
答：成功加载。但是无结构（schema unknown），discribe后看到：Schema for origin_cleaned_data unknown。
这种情况类似union，合并两个不同列的对象，会生成一个未知模式对象。
背景：因为老日志42列，新日志多加一列在第20列，因为20列后面不能同名，又要总体日志的用户点击数。所以一起加载，统一统计。
（如果知道不同日期日志的类型，则可以分别读入，指定明确模式，然后用onschema进行uion，在分别统计。可惜接受项目，不确定线上哪天改的）
采样：老日志log_without.txt，新日志log_with_android_ad_id.txt
代码如下
REGISTER piggybank.jar;
DEFINE SequenceFileLoader org.apache.pig.piggybank.storage.SequenceFileLoader();
%default cleanedLog /user/wizad/tmp/log_*
--%default cleanedLog1 /home/wizad/lmj/log_without.txt
--%default cleanedLog 2/home/wizad/lmj/log_with_android_ad_id.txt
origin_cleaned_data = LOAD '$cleanedLog' USING PigStorage(',');
DUMP origin_cleaned_data;
DESCRIBE origin_cleaned_data;
显示结果：
((null) 5,74,48809e40-b8d7-41a4-bf68-d0f8e28140ad,575356365101899146,2014-07-30 10:33:56,2014-07-30 10:33:56,1,57074,2,,,,,,,,151.87.202.1,1,-1,-1,lmj,-1,1ac2c73e-d93a-4801-a7ee-da05473d0585,48809e40-b8d7-41a4-bf68-d0f8e28140ad,02:00:00:00:00:00,1940064625594046032,,,,d70cc494,25100,206,,0,2,2,7.1,,,,42.833298,12.833298,120232210032202)
((null) 5,74,357633052513139,1033882907630785616,2014-07-30 11:15:05,2014-07-30 11:15:05,1,57074,2,,,,,,,,155.128.32.119,1,357633052513139,270f213575a4eda7,lmj,270f213575a4eda7,,,40:0e:85:40:0e:1a,-7537294162085162169,,,,7626e397,62713,206,,2,1,3,4.3,,,,37.774902,-122.4194,023010203333003)
((null) 5,74,e7a4afce-ffd9-4ecd-b916-39f9d793c218,207640323432175503,2014-07-30 10:29:22,2014-07-30 10:29:22,1,57074,2,,,,,,,,111.200.142.163,1,-1,-1,lmj,-1,14ea5e95237f34e278d7ac210173d6b8ad9d5026,e7a4afce-ffd9-4ecd-b916-39f9d793c218,02:00:00:00:00:00,1179719885610920154,,,,d4eeab6e,66104,101,,0,2,2,7.1,1,7,7,39.928894,116.388306,132100103322203)
((null) 5,74,48809e40-b8d7-41a4-bf68-d0f8e28140ad,575356365101899146,2014-07-30 10:33:56,2014-07-30 10:33:56,1,57074,2,,,,,,,,151.87.202.1,1,-1,-1,-1,1ac2c73e-d93a-4801-a7ee-da05473d0585,48809e40-b8d7-41a4-bf68-d0f8e28140ad,02:00:00:00:00:00,1940064625594046032,,,,d70cc494,25100,206,,0,2,2,7.1,,,,42.833298,12.833298,120232210032202)
((null) 5,74,302bd8f1-b974-4af5-8183-1f67d27410d6,367366268601246781,2014-07-30 10:07:57,2014-07-30 10:07:57,1,57074,2,,,,,,,,56.2.255.220,1,-1,-1,-1,c165376f9f76cf68862a505328b7ba7cd0cfa0b0,302bd8f1-b974-4af5-8183-1f67d27410d6,02:00:00:00:00:00,-488564527359896578,,,,103b14d3,25100,206,,0,2,2,7.1,,,,37.774902,-122.4194,023010203333003)
((null) 5,74,e7a4afce-ffd9-4ecd-b916-39f9d793c218,207640323432175503,2014-07-30 10:29:22,2014-07-30 10:29:22,1,57074,2,,,,,,,,111.200.142.163,1,-1,-1,-1,14ea5e95237f34e278d7ac210173d6b8ad9d5026,e7a4afce-ffd9-4ecd-b916-39f9d793c218,02:00:00:00:00:00,1179719885610920154,,,,d4eeab6e,66104,101,,0,2,2,7.1,1,7,7,39.928894,116.388306,132100103322203)
Schema for origin_cleaned_data unknown.
多了一列值为lmj的列。可以看到无结构。
union：合并不同格式的列
（union不去重复行）
A = load 'input1' as (x:int, y:float);
B = load 'input2' as (x:int, y:chararray);
C = union A, B;
describe C;
显示结果：
Schema for C unknown
两个不通列名的变量union用onschema
需要注意：使用onschema，需要所有的输入都要有明确的schema，否则，错误。因为union合并时，比较是按名字和列类型（可以自动从低级到高级类型转换）。
合并后，空出的列会补null。
A = load 'input1' as (w: chararray, x:int, y:float);
B = load 'input2' as (x:int, y:double, z:chararray);
C = union onschema A, B;
describe C;
结果：
C: {w: chararray,x: int,y: double,z: chararray}
给出一个不能union的代码例子
%default cleanedLog1 /home/wizad/lmj/log_without.txt
%default cleanedLog2 /home/wizad/lmj/log_with_android_ad_id.txt
origin1 = LOAD '$cleanedLog1' USING PigStorage(',');
origin2 = LOAD '$cleanedLog2' USING PigStorage(',');
DESCRIBE origin1
DESCRIBE origin2
origin = union origin1,origin2
结果：
origin1和origin2显示Schema for origin2 unknown.
所以origin不能生成