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https://issues.apache.org/jira/browse/HIVE-2146?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13028563#comment-13028563
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Siying Dong commented on HIVE-2146:
-----------------------------------
review board:
https://reviews.apache.org/r/685/
> Block Sampling should adjust number of reducers accordingly to make it useful
> -----------------------------------------------------------------------------
>
> Key: HIVE-2146
> URL: https://issues.apache.org/jira/browse/HIVE-2146
> Project: Hive
> Issue Type: Bug
> Reporter: Siying Dong
> Assignee: Siying Dong
> Attachments: HIVE-2146.1.patch, HIVE-2146.2.patch
>
>
> Now number of reducers of block sampling is not modified, so that queries
> like:
> select c from tab tablesample(1 percent) group by c;
> can generate huge number of reducers although the input is sampled to be
> small.
> We need to shrink number of reducers to make block sampling more useful.
> Since now number of reducers are determined before get splits, the way to do
> it probably is not clean enough, but we can do a good guess.
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