[jira] [Commented] (HIVE-4002) Fetch task aggregation for simple group by query

["Edward Capriolo (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-4002?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13748576#comment-13748576 ] 

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

+1 this is a very exciting feature. Will commit when tests pass.
                
> 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, 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,
> 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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