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From "Prasanth Jayachandran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-15477) Provide options to adjust filter stats when column stats are not available
Date Wed, 21 Dec 2016 21:55:58 GMT

    [ https://issues.apache.org/jira/browse/HIVE-15477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15768281#comment-15768281
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Prasanth Jayachandran commented on HIVE-15477:
----------------------------------------------

Looks like HIVE-11110 already fixed this mis-estimation. Before that it used to consider the
IS NOT NULL predicate as constant and reduce the rows by half. Sorry I lost track of this
change that fixed it.

> Provide options to adjust filter stats when column stats are not available
> --------------------------------------------------------------------------
>
>                 Key: HIVE-15477
>                 URL: https://issues.apache.org/jira/browse/HIVE-15477
>             Project: Hive
>          Issue Type: Bug
>          Components: Statistics
>    Affects Versions: 2.2.0
>            Reporter: Chao Sun
>            Assignee: Chao Sun
>         Attachments: HIVE-15477.1.patch
>
>
> Currently when column stats are not available, Hive will assume the "worst" case by setting
the # of output rows to be 1/2 of the # of input rows, for each predicate expression. This
could be inaccurate, especially in the presence of multiple predicates chained by AND. We
have found in some cases this could cause map join to have wrong ordering and thus fail with
memory issue.
> One suggestion is to provide a config (such as {{hive.stats.filter.factor}}) that can
be used to control the percentage of rows emitted by a predicate expression. 



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