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From "He Yongqiang (Updated) (JIRA)" <>
Subject [jira] [Updated] (HIVE-2621) Allow multiple group bys with the same input data and spray keys to be run on the same reducer.
Date Tue, 03 Jan 2012 18:10:39 GMT


He Yongqiang updated HIVE-2621:

    Resolution: Fixed
        Status: Resolved  (was: Patch Available)

committed, thanks Kevin!
> Allow multiple group bys with the same input data and spray keys to be run on the same
> -----------------------------------------------------------------------------------------------
>                 Key: HIVE-2621
>                 URL:
>             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, HIVE-2621.D567.4.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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