Hadoop: the definitive guide 第三版 拾遗 第十章 之Pig

概述：

Pig的安装很简单，注意一下几点：

1、设置系统环境变量：

export PIG_HOME=.../pig-x.y.z
export PATH=$PATH:$PIG_HOME/bin

设置完成后使用pig -help进行验证一下。

2、两种mode：

local mode：访问本地文件系统，进入shell时使用命令：pig -x local

MapReduce mode：pig将查询翻译为MapReduce作业，然后在hadoop集群上执行。此时，进入shell时的命令为：pig -x mapreduce 或者pig

hadoop@master:/usr/local/hadoop/conf$ pig -x mapreduce
Warning: $HADOOP_HOME is deprecated.

2013-08-16 16:18:52,388 [main] INFO  org.apache.pig.Main - Apache Pig version 0.11.1 (r1459641) compiled Mar 22 2013, 02:13:53
2013-08-16 16:18:52,389 [main] INFO  org.apache.pig.Main - Logging error messages to: /usr/local/hadoop/conf/pig_1376641132384.log
2013-08-16 16:18:52,470 [main] INFO  org.apache.pig.impl.util.Utils - Default bootup file /home/hadoop/.pigbootup not found
2013-08-16 16:18:52,760 [main] INFO  org.apache.pig.backend.hadoop.executionengine.HExecutionEngine - Connecting to hadoop file system at: hdfs://master:9000
2013-08-16 16:18:53,174 [main] INFO  org.apache.pig.backend.hadoop.executionengine.HExecutionEngine - Connecting to map-reduce job tracker at: master:9001

注意：使用MapReduce模式需要设置hadoop的配置文件hadoop-env.sh，加入：

export PIG_CLASSPATH=$HADOOP_HOME/conf

示例一：

.../in/ncdc/micro-tab/sample.txt文件的内容为：

1950	0	1
1950	22	1
1950	-11	1
1949	111	1
1949	78	1

在pig的shell下执行下列命令：

grunt> -- max_temp.pig: Finds the maximum temperature by year
grunt> records = LOAD 'hdfs://master:9000/in/ncdc/micro-tab/sample.txt'--在不确定自己设置的默认路径是什么的情况下使用hdfs完整路径
>>   AS (year:chararray, temperature:int, quality:int);
grunt> filtered_records = FILTER records BY temperature != 9999 AND
>>   (quality == 0 OR quality == 1 OR quality == 4 OR quality == 5 OR quality == 9);
grunt> grouped_records = GROUP filtered_records BY year;
grunt> max_temp = FOREACH grouped_records GENERATE group,
>>   MAX(filtered_records.temperature);
grunt> DUMP max_temp;

pig同时提供ILLUSTRATE操作，以生成简洁明了的数据集。

grunt>ILLUSTRATE max_temp;

输出为：

示例二：

指南中关于注释的示例，在此处，略作修改，加入schema：

grunt> B = LOAD 'input/pig/join/B' AS (chararry,int);
grunt> A = LOAD 'input/pig/join/A' AS (int,chararry);
grunt> C = JOIN A BY $0, /* ignored */ B BY $1;
grunt> DESCRIBE C
C: {A::val_0: int,A::chararry: bytearray,B::chararry: bytearray,B::val_0: int}
grunt>  ILLUSTRATE C 

输出为：

----------------------------------------------------
| A     | val_0:int      | chararry:bytearray      |
----------------------------------------------------
|       | 3              | Hat                     |
|       | 3              | Hat                     |
----------------------------------------------------
----------------------------------------------------
| B     | chararry:bytearray      | val_0:int      |
----------------------------------------------------
|       | Eve                     | 3              |
|       | Eve                     | 3              |
----------------------------------------------------
-----------------------------------------------------------------------------------------------------------
| C     | A::val_0:int      | A::chararry:bytearray      | B::chararry:bytearray      | B::val_0:int      |
-----------------------------------------------------------------------------------------------------------
|       | 3                 | Hat                        | Eve                        | 3                 |
|       | 3                 | Hat                        | Eve                        | 3                 |
|       | 3                 | Hat                        | Eve                        | 3                 |
|       | 3                 | Hat                        | Eve                        | 3                 |
-----------------------------------------------------------------------------------------------------------

注意：Pig Latin的大小写敏感性采用混合的规则，其中：

操作和命令是大小写无关；

别名和函数大小写敏感。

例如上例中：

grunt> describe c
2013-08-16 17:14:49,397 [main] ERROR org.apache.pig.tools.grunt.Grunt - ERROR 1005: No plan for c to describe
Details at logfile: /usr/local/hadoop/conf/pig_1376641755235.log
grunt> describe C
C: {A::val_0: int,A::chararry: bytearray,B::chararry: bytearray,B::val_0: int}
grunt> DESCRIBE C
C: {A::val_0: int,A::chararry: bytearray,B::chararry: bytearray,B::val_0: int}