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Date: Tue, 9 Jul 2013 03:59:48 +0000 (UTC)
From: "Edward Capriolo (JIRA)" <jira@apache.org>
To: hive-dev@hadoop.apache.org
Message-ID: <JIRA.12631407.1360314190199.16294.1373342388648@arcas>
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Subject: [jira] [Commented] (HIVE-4002) Fetch task aggregation for simple
 group by query
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    [ https://issues.apache.org/jira/browse/HIVE-4002?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13702867#comment-13702867 ] 

Edward Capriolo commented on HIVE-4002:
---------------------------------------

Testing now. The threshold can be a follow on. I will do a more critical review in the next couple of days.
                
> Fetch task aggregation for simple group by query
> ------------------------------------------------
>
>                 Key: HIVE-4002
>                 URL: https://issues.apache.org/jira/browse/HIVE-4002
>             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
>
>
> 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,
> TS-FIL-SEL-GBY1-RS-GBY2-SEL-FS + FETCH-TASK
> into 
> TS-FIL-SEL-GBY1-FS + FETCH-TASK(GBY2-SEL-LS)
> With the patch, time taken for auto_join_filters.q test reduced to 6 min (10 min, before).

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