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Date: Wed, 27 Nov 2013 05:30:42 +0000 (UTC)
From: "Sun Rui (JIRA)" <jira@apache.org>
To: hive-dev@hadoop.apache.org
Message-ID: <JIRA.12681234.1385460645376.36505.1385530242656@arcas>
In-Reply-To: <JIRA.12681234.1385460645376@arcas>
References: <JIRA.12681234.1385460645376@arcas>
Subject: [jira] [Commented] (HIVE-5891) Alias conflict when merging multiple
 mapjoin tasks into their common child mapred task
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    [ https://issues.apache.org/jira/browse/HIVE-5891?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13833477#comment-13833477 ] 

Sun Rui commented on HIVE-5891:
-------------------------------

[~yhuai] Yes, "$JOIN_INTERMEDIATE" is intended to name an intermediate dataset during the processing of join, not for input dataset. Is there anything wrong with my understanding?  "$JOIN_STREAM" is also another candidate maybe.

GenMapRedUtils.java:
{code}
  private static boolean needsTagging(ReduceWork rWork) {
    return rWork != null && (rWork.getReducer().getClass() == JoinOperator.class ||
         rWork.getReducer().getClass() == DemuxOperator.class);
  }

splitTasks():
    if (needsTagging(cplan.getReduceWork())) {
      String origStreamDesc;
      Operator<? extends OperatorDesc> joinOp = cplan.getReduceWork().getReducer();
      QBJoinTree joinTree = parseCtx.getJoinContext().get(joinOp);
      if (joinTree != null) {
        streamDesc = joinTree.getJoinStreamDesc();
      } else {
        streamDesc = "$INTNAME";
      }
{code}

Look at the above piece of code, an alias is also needed when the reduce work starts
with a DemuxOperator. For this case, "$INTNAME" is still un-changed because I have no
idea how DemuxOperator works.

> Alias conflict when merging multiple mapjoin tasks into their common child mapred task
> --------------------------------------------------------------------------------------
>
>                 Key: HIVE-5891
>                 URL: https://issues.apache.org/jira/browse/HIVE-5891
>             Project: Hive
>          Issue Type: Bug
>          Components: Query Processor
>    Affects Versions: 0.12.0
>            Reporter: Sun Rui
>            Assignee: Sun Rui
>         Attachments: HIVE-5891.1.patch
>
>
> Use the following test case with HIVE 0.12:
> {quote}
> create table src(key int, value string);
> load data local inpath 'src/data/files/kv1.txt' overwrite into table src;
> select * from (
>   select c.key from
>     (select a.key from src a join src b on a.key=b.key group by a.key) tmp
>     join src c on tmp.key=c.key
>   union all
>   select c.key from
>     (select a.key from src a join src b on a.key=b.key group by a.key) tmp
>     join src c on tmp.key=c.key
> ) x;
> {quote}
> We will get a NullPointerException from Union Operator:
> {quote}
> java.lang.RuntimeException: org.apache.hadoop.hive.ql.metadata.HiveException: Hive Runtime Error while processing row {"_col0":0}
> 	at org.apache.hadoop.hive.ql.exec.mr.ExecMapper.map(ExecMapper.java:175)
> 	at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)
> 	at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:436)
> 	at org.apache.hadoop.mapred.MapTask.run(MapTask.java:372)
> 	at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:212)
> Caused by: org.apache.hadoop.hive.ql.metadata.HiveException: Hive Runtime Error while processing row {"_col0":0}
> 	at org.apache.hadoop.hive.ql.exec.MapOperator.process(MapOperator.java:544)
> 	at org.apache.hadoop.hive.ql.exec.mr.ExecMapper.map(ExecMapper.java:157)
> 	... 4 more
> Caused by: java.lang.NullPointerException
> 	at org.apache.hadoop.hive.ql.exec.UnionOperator.processOp(UnionOperator.java:120)
> 	at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:504)
> 	at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:842)
> 	at org.apache.hadoop.hive.ql.exec.SelectOperator.processOp(SelectOperator.java:88)
> 	at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:504)
> 	at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:842)
> 	at org.apache.hadoop.hive.ql.exec.CommonJoinOperator.genUniqueJoinObject(CommonJoinOperator.java:652)
> 	at org.apache.hadoop.hive.ql.exec.CommonJoinOperator.genUniqueJoinObject(CommonJoinOperator.java:655)
> 	at org.apache.hadoop.hive.ql.exec.CommonJoinOperator.checkAndGenObject(CommonJoinOperator.java:758)
> 	at org.apache.hadoop.hive.ql.exec.MapJoinOperator.processOp(MapJoinOperator.java:220)
> 	at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:504)
> 	at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:842)
> 	at org.apache.hadoop.hive.ql.exec.TableScanOperator.processOp(TableScanOperator.java:91)
> 	at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:504)
> 	at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:842)
> 	at org.apache.hadoop.hive.ql.exec.MapOperator.process(MapOperator.java:534)
> 	... 5 more
> {quote}
>   
> The root cause is in CommonJoinTaskDispatcher.mergeMapJoinTaskIntoItsChildMapRedTask().
>   +--------------+      +--------------+
>   | MapJoin task |      | MapJoin task |
>   +--------------+      +--------------+
>              \             /
>               \           /
>              +--------------+
>              |  Union task  |
>              +--------------+
>              
> CommonJoinTaskDispatcher merges the two MapJoin tasks into their common child: Union task. The two MapJoin tasks have the same alias name for their big tables: $INTNAME, which is the name of the temporary table of a join stream. The aliasToWork map uses alias as key, so eventually only the MapJoin operator tree of one MapJoin task is saved into the aliasToWork map of the Union task, while the MapJoin operator tree of another MapJoin task is lost. As a result, Union operator won't be initialized because not all of its parents gets intialized (The Union operator itself indicates it has two parents, but actually it has only 1 parent because another parent is lost).
> This issue does not exist in HIVE 0.11 and thus is a regression bug in HIVE 0.12.
> The propsed solution is to use the query ID as prefix for the join stream name to avoid conflict and add sanity check code in CommonJoinTaskDispatcher that merge of a MapJoin task into its child MapRed task is skipped if there is any alias conflict. Please review the patch. I am not sure if the patch properly handles the case of DemuxOperator.
> BTW, anyone knows the origin of "$INTNAME"? it is so confusing, maybe we can replace it with a meaningful name.


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