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From "Ashutosh Chauhan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-21398) Columns which has estimated statistics should not be considered as unique keys
Date Thu, 07 Mar 2019 22:28:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-21398?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16787273#comment-16787273
] 

Ashutosh Chauhan commented on HIVE-21398:
-----------------------------------------

+1

> Columns which has estimated statistics should not be considered as unique keys
> ------------------------------------------------------------------------------
>
>                 Key: HIVE-21398
>                 URL: https://issues.apache.org/jira/browse/HIVE-21398
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Zoltan Haindrich
>            Assignee: Zoltan Haindrich
>            Priority: Major
>         Attachments: HIVE-21398.01.patch
>
>
> Right now for a column to qualify as a unique column it has to meet the criteria: 
> {code}
> NDV >= numRows
> {code}
> when numRows is 1 this tends to be true ; but numRows is also 1 in cases when we are
kinda operate in the blind - don't know how many row there are - more generatlly: with estimated
column statistics.
> As a sideeffect of qualifying all columns to be unique; after a few joins all column
combinations became unique....so for a join between 3 tables which have (i,j,k) columns; then
it will allocate {{i*j*k}} triplets of "unique column triplets".



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