Pig:Relation 和 Schema 名称混淆

Posted 2023-04-18

【中文标题】Pig:Relation 和 Schema 名称混淆

【英文标题】：Pig:Relation and Schema name confusion

【发布时间】：2015-01-31 16:15:25

【问题描述】：

在 Pig Latin 中；这按预期工作：

filtered = FILTER records BY age > 27;

但这会引发异常（当 >> DUMP 过滤时）：

filtered = FILTER records BY records.age > 27;

这是例外：

java.lang.Exception: org.apache.pig.backend.executionengine.ExecException: ERROR 0: Scalar has more than one row in the output. 1st : (John,Wilk,27,M), 2nd :(Tri,Tim,27,F)
    at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:403)
Caused by: org.apache.pig.backend.executionengine.ExecException: ERROR 0: Scalar has more than one row in the output. 1st : (John,Wilk,27,M), 2nd :(Tri,Tim,27,F)
    at org.apache.pig.impl.builtin.ReadScalars.exec(ReadScalars.java:119)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POUserFunc.getNext(POUserFunc.java:345)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POUserFunc.getNextInteger(POUserFunc.java:394)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.PhysicalOperator.getNext(PhysicalOperator.java:322)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.GreaterThanExpr.getNextBoolean(GreaterThanExpr.java:74)
    at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POFilter.getNextTuple(POFilter.java:144)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.runPipeline(PigGenericMapBase.java:282)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.map(PigGenericMapBase.java:277)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapBase.map(PigGenericMapBase.java:64)
    at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:145)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:763)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:339)
    at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:235)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
    at java.util.concurrent.FutureTask.run(FutureTask.java:262)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:744)

两者有什么区别？它们不一样吗？

【参考方案1】：

不，两个 stmts 是不同的。

第一个 stmt 完全有效，在这种情况下，pig 将遍历每一行并应用过滤器约束（年龄 > 27）。它是使用过滤器 stmts 的标准方式。

在第二种情况下，你使用dereference operator(.)来访问字段，但是当你使用解引用操作符访问时，解引用操作符主要用于访问复杂数据类型（Tuples、Bags和Maps）的值然后猪的字段将always expect the scalar output（即过滤条件后只有一个输出）不幸的是你的过滤条件（年龄> 27）返回more than one matching result，这就是你得到“Scalar has more than one row in the output”的原因 如果您的过滤条件（年龄>27）只返回一个输出，那么您的 stmt 是完全有效的。

【讨论】：

感谢您的解释。