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From "Phabricator (JIRA)" <>
Subject [jira] [Commented] (HIVE-4002) Fetch task aggregation for simple group by query
Date Tue, 27 Aug 2013 15:04:52 GMT


Phabricator commented on HIVE-4002:

yhuai has commented on the revision "HIVE-4002 [jira] Fetch task aggregation for simple group
by query".

  ql/src/java/org/apache/hadoop/hive/ql/exec/ I did not mean we cannot have
a recursive flush method. I meant that Demux and Mux operators should not use a recursive
flush method.


To: JIRA, navis
Cc: yhuai

> Fetch task aggregation for simple group by query
> ------------------------------------------------
>                 Key: HIVE-4002
>                 URL:
>             Project: Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Navis
>            Assignee: Navis
>            Priority: Minor
>         Attachments: HIVE-4002.D8739.1.patch, HIVE-4002.D8739.2.patch, HIVE-4002.D8739.3.patch,
> Aggregation queries with no group-by clause (for example, select count(*) from src) executes
final aggregation in single reduce task. But it's too small even for single reducer because
the most of UDAF generates just single row for map aggregation. If final fetch task can aggregate
outputs from map tasks, shuffling time can be removed.
> This optimization transforms operator tree something like,
> into 
> With the patch, time taken for auto_join_filters.q test reduced to 6 min (10 min, before).

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