问题描述
我正在独自学习Pig，在尝试探索数据集时遇到了异常.脚本中有什么问题以及原因:
I am learning Pig on my own and while trying to explore a dataset I am encountering an exception. What is wrong in the script and why:
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.
At the end of the MAP Reduce execution I am getting the below error.
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" )
推荐答案
首先，让我们看看如何解决您的问题.您无需使用别名访问您的字段.您的第三行可能很简单:
First, let's see how we can fix your problem. You don't need to access your fields using the alias name. Your third line could be simply:
high_rated = FOREACH high GENERATE title, year, rating, duration;
如果出于某种原因要使用别名，则应使用引用运算符(::)，如ERROR建议所示.然后您的行将如下所示:
If you wanted to use the alias name for some reason you should use the referential operator (::) as can be seen in the ERROR suggestion. Then your line would look like:
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将假定别名为标量(别名只有一行).由于您的别名有多行，因此抱怨.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
Next, let's try to understand the exact reason behind the error message. When you try to access the fields using a dot operator (.), pig will assume that the alias is a scalar (alias having only one row). Since your alias had more than one row, it complained. You can read more about scalars in Pig here: https://issues.apache.org/jira/browse/PIG-1434
在JIRA的发行说明部分中，您会在最后注意到，预期的错误消息与您得到的错误相匹配:
In the JIRA's release notes section, you will notice at the end, the expected error message matches the error you are getting:
If a relation contains more than single tuple, a runtime error is generated:
"Scalar has more than one row in the output"
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