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From "Ning Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-2146) Block Sampling should adjust number of reducers accordingly to make it useful
Date Wed, 04 May 2011 00:46:03 GMT

    [ https://issues.apache.org/jira/browse/HIVE-2146?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13028532#comment-13028532
] 

Ning Zhang commented on HIVE-2146:
----------------------------------

Siying can you create a review request? 

I've a couple of comments as well: 
 1) comment in line 387 is not complete
 2) comments in line 388-389: can you give an example in which case all input alias are sampled
by syntax but it actually cannot be sampled?
 3) line 391: if we want to mimic the old estimation algorithm it seems we shouldn't + bytesPerReducer
here? It is added in line 399 right?  

> 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
>
>
> 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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