尝试执行 Pig Latin 脚本时出现异常

Question

我正在自学 Pig，在尝试探索数据集时遇到异常。脚本有什么问题以及原因：

movies_data = LOAD '/movies_data' using PigStorage(',') as (id:chararray,title:chararray,year:int,rating:double,duration:double);
high   = FILTER movies_data by rating > 4.0;
high_rated = FOREACH high GENERATE movies_data.title,movies_data.year,movies_data.rating,movies_data.duration;
DUMP high_rated;

在 MAP Reduce 执行结束时，出现以下错误。

2018-07-22 20:11:07,213 [main] ERROR org.apache.pig.tools.grunt.Grunt

ERROR 1066: Unable to open iterator for alias high_rated. 
Backend error : org.apache.pig.backend.executionengine.ExecException: 
ERROR 0: Scalar has more than one row in the output. 
1st : (1,The Nightmare Before Christmas,1993,3.9,4568.0), 
2nd :(2,The Mummy,1932,3.5,4388.0) 
(common cause: "JOIN" then "FOREACH ... GENERATE foo.bar" should be "foo::bar" )

Answer 1

首先，让我们看看如何解决您的问题。您不需要使用别名访问您的字段。您的第三行可以是：

high_rated = FOREACH high GENERATE title, year, rating, duration;

如果您出于某种原因想要使用别名，您应该使用引用运算符 (::)，如错误建议中所示。然后你的行看起来像：

high_rated = FOREACH high GENERATE movies_data::title, movies_data::year, movies_data::rating, movies_data::duration;

接下来，让我们尝试了解错误消息背后的确切原因。当您尝试使用点运算符 (.) 访问字段时，pig 会假定别名是标量（别名只有一行）。由于您的别名不止一行，因此它会抱怨。您可以在此处阅读有关 Pig 中标量的更多信息：https://issues.apache.org/jira/browse/PIG-1434

在 JIRA 的发行说明部分，您会在末尾注意到，预期的错误消息与您收到的错误相符：

If a relation contains more than single tuple, a runtime error is generated: 
"Scalar has more than one row in the output"

Answer 2

这对你来说没有错误。

movies_data = LOAD '/movies_data' using PigStorage(',') as (id:chararray,title:chararray,year:int,rating:double,duration:double);
high   = FILTER movies_data by rating > 4.0;
 high_rated = FOREACH high GENERATE title,year,rating,duration;
DUMP high_rated;

FILTER命令允许所有满足过滤条件的列记录。

尝试执行 Pig Latin 脚本时出现异常

Getting exception while trying to execute a Pig Latin Script

hadoop

mapreduce

apache-pig