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https://issues.apache.org/jira/browse/HIVE-7045?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14053854#comment-14053854
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Ashutosh Chauhan commented on HIVE-7045:
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+1
> Wrong results in multi-table insert aggregating without group by clause
> -----------------------------------------------------------------------
>
> Key: HIVE-7045
> URL: https://issues.apache.org/jira/browse/HIVE-7045
> Project: Hive
> Issue Type: Bug
> Affects Versions: 0.10.0, 0.12.0
> Reporter: dima machlin
> Assignee: Navis
> Priority: Blocker
> Attachments: HIVE-7045.1.patch.txt
>
>
> This happens whenever there are more than 1 reducers.
> The scenario :
> CREATE TABLE t1 (a int, b int);
> CREATE TABLE t2 (cnt int) PARTITIONED BY (var_name string);
> insert into table t1 select 1,1 from asd limit 1;
> insert into table t1 select 2,2 from asd limit 1;
> t1 contains :
> 1 1
> 2 2
> from t1
> insert overwrite table t2 partition(var_name='a') select count(a) cnt
> insert overwrite table t2 partition(var_name='b') select count(b) cnt ;
> select * from t2;
> returns :
> 2 a
> 2 b
> as expected.
> Setting the number of reducers higher than 1 :
> set mapred.reduce.tasks=2;
> from t1
> insert overwrite table t2 partition(var_name='a') select count(a) cnt
> insert overwrite table t2 partition(var_name='b') select count(b) cnt;
> select * from t2;
> 1 a
> 1 a
> 1 b
> 1 b
> Wrong results.
> This happens when ever t1 is big enough to automatically generate more than 1
> reducers and without specifying it directly.
> adding "group by 1" in the end of each insert solves the problem :
> from t1
> insert overwrite table t2 partition(var_name='a') select count(a) cnt group
> by 1
> insert overwrite table t2 partition(var_name='b') select count(b) cnt group
> by 1;
> generates :
> 2 a
> 2 b
> This should work without the group by...
> The number of rows for each partition will be the amount of reducers.
> Each reducer calculated a sub total of the count.
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