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From "Prasanth Jayachandran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-9392) JoinStatsRule miscalculates join cardinality as incorrect NDV is used due to column names having duplicated fqColumnName
Date Thu, 22 Jan 2015 02:24:36 GMT

    [ https://issues.apache.org/jira/browse/HIVE-9392?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14286824#comment-14286824
] 

Prasanth Jayachandran commented on HIVE-9392:
---------------------------------------------

This patch uses the tag number in fully qualified column name to disambiguate between same
key column names. 

> JoinStatsRule miscalculates join cardinality as incorrect NDV is used due to column names
having duplicated fqColumnName
> ------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HIVE-9392
>                 URL: https://issues.apache.org/jira/browse/HIVE-9392
>             Project: Hive
>          Issue Type: Bug
>          Components: Physical Optimizer
>    Affects Versions: 0.14.0
>            Reporter: Mostafa Mokhtar
>            Assignee: Prasanth Jayachandran
>            Priority: Critical
>             Fix For: 0.15.0
>
>         Attachments: HIVE-9392.1.patch
>
>
> In JoinStatsRule.process the join column statistics are stored in HashMap  joinedColStats,
the key used which is the ColStatistics.fqColName is duplicated between join column in the
same vertex, as a result distinctVals ends up having duplicated values which negatively affects
the join cardinality estimation.
> The duplicate keys are usually named KEY.reducesinkkey0.



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