标量只能用于PIG中的投影 [英] scalars can only be used with projection in PIG

问题描述

标量只能用于投影我使用foreach时出现这个错误。我该如何解决这个错误？我如何在foreach中使用LIMIT？请提前建议一些
。

编辑（Tichdroma）：从评论中复制代码

  A = LOAD'part-r-00000'; 
 G = A组乘以（$ 0，$ 2）; 
 Y = foreach G生成FLATTEN（组），FLATTEN（$ 1）; 
 sorted =订购Y $ 0 ASC，$ 1 DESC; 
 X = foreach Y {
 lim = LIMIT sorted 3; 
生成lim; 
}; 
转储x; 
  

解决方案

LIMIT 可在FOREACH的Pig 0.9中找到 nested_op 。

如果您想要每个组的前N个元素，你可能想尝试迭代每一个并单独排序和限制它们：

  A = LOAD'part-r-00000 ; 
 G = GROUP A by（$ 0，$ 2）; 
 X = FOREACH G {
 sorted = ORDER A by $ 0 ASC，$ 1 DESC; 
 lim = LIMIT排序3; 
 GENERATE lim; 
}; 
 DUMP X; 
  

请注意 TOP 可以很有效率，只要有一列可比较的值（不是这种情况下）。

scalars can only be used with projection i am getting this error while using foreach.How can i resolved this error ? how can i use LIMIT within foreach ? please suggest some thanks in advance..

Edit (Tichdroma): Copied code from comment

A = LOAD 'part-r-00000';
G = Group A by ($0,$2 );
Y = foreach G generate FLATTEN(group), FLATTEN($1);
sorted = order Y by $0 ASC, $1 DESC;
X = foreach Y {
  lim = LIMIT sorted 3;
  generate lim;
};
Dump x;

解决方案

LIMIT is available in Pig 0.9 in the FOREACH nested_op.

If you want the top N element of each group, you might want to try to iterate on each one and individually sort and limit them:

A = LOAD 'part-r-00000';
G = GROUP A by ($0, $2);
X = FOREACH G {
  sorted = ORDER A by $0 ASC, $1 DESC;
  lim = LIMIT sorted 3;
  GENERATE lim;
};
DUMP X;

Notice that TOP can be efficient when you just have a column of comparable values (not in this case).

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