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From "Phabricator (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-2621) Allow multiple group bys with the same input data and spray keys to be run on the same reducer.
Date Thu, 22 Dec 2011 17:43:31 GMT

    [ https://issues.apache.org/jira/browse/HIVE-2621?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174922#comment-13174922
] 

Phabricator commented on HIVE-2621:
-----------------------------------

njain has commented on the revision "HIVE-2621 [jira] Allow multiple group bys with the same
input data and spray keys to be run on the same reducer.".

  otherwise it looks good

INLINE COMMENTS
  ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java:5925 can you give an example
in the comments ?

  Sorry, but it is not clear to me.

  Do you want to return 2 lists - one for the common distincts ?
  I am missing something: what else do you want to return ?


  ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java:3469 thanks for creating
this function and re-using this code

REVISION DETAIL
  https://reviews.facebook.net/D567

                
> Allow multiple group bys with the same input data and spray keys to be run on the same
reducer.
> -----------------------------------------------------------------------------------------------
>
>                 Key: HIVE-2621
>                 URL: https://issues.apache.org/jira/browse/HIVE-2621
>             Project: Hive
>          Issue Type: New Feature
>            Reporter: Kevin Wilfong
>            Assignee: Kevin Wilfong
>         Attachments: HIVE-2621.1.patch.txt, HIVE-2621.D567.1.patch, HIVE-2621.D567.2.patch,
HIVE-2621.D567.3.patch
>
>
> Currently, when a user runs a query, such as a multi-insert, where each insertion subclause
consists of a simple query followed by a group by, the group bys for each clause are run on
a separate reducer.  This requires writing the data for each group by clause to an intermediate
file, and then reading it back.  This uses a significant amount of the total CPU consumed
by the query for an otherwise simple query.
> If the subclauses are grouped by their distinct expressions and group by keys, with all
of the group by expressions for a group of subclauses run on a single reducer, this would
reduce the amount of reading/writing to intermediate files for some queries.
> To do this, for each group of subclauses, in the mapper we would execute a the filters
for each subclause 'or'd together (provided each subclause has a filter) followed by a reduce
sink.  In the reducer, the child operators would be each subclauses filter followed by the
group by and any subsequent operations.
> Note that this would require turning off map aggregation, so we would need to make using
this type of plan configurable.

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