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https://issues.apache.org/jira/browse/HIVE-4002?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14052987#comment-14052987
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Lefty Leverenz commented on HIVE-4002:
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*hive.fetch.task.aggr* is documented in the wiki here:
* [Configuration Properties -- hive.fetch.task.aggr |
https://cwiki.apache.org/confluence/display/Hive/Configuration+Properties#ConfigurationProperties-hive.fetch.task.aggr]
Also see doc comments on HIVE-5793 (Update hive-default.xml.template for
HIVE-4002).
> 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
> Fix For: 0.12.0
>
> Attachments: HIVE-4002.D8739.1.patch, HIVE-4002.D8739.2.patch,
> HIVE-4002.D8739.3.patch, HIVE-4002.D8739.4.patch, HIVE-4002.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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