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From "Sergey Shelukhin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-15397) metadata-only queries may return incorrect results with empty tables
Date Fri, 09 Dec 2016 21:47:58 GMT

    [ https://issues.apache.org/jira/browse/HIVE-15397?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15736432#comment-15736432
] 

Sergey Shelukhin commented on HIVE-15397:
-----------------------------------------

Interesting q file changes.. according to our take on 1=1 group by 1=1 they are correct.
E.g. table has 3 partitions, part=a, part=b, and part=c. Only a and c have data.
select distinct part from t
used to return "a, b, c". However, there are no rows in the table that actually have value
b. So, the result has changed to "a, c".
[~ashutoshc] [~jcamachorodriguez] would you say it's the correct change and previous result
is incorrect?
Same for max(partcol) from an empty table - should it be null? Cause there are no rows in
the table to derive max from, similar how there are no rows in gby 1=1 to group by.

> metadata-only queries may return incorrect results with empty tables
> --------------------------------------------------------------------
>
>                 Key: HIVE-15397
>                 URL: https://issues.apache.org/jira/browse/HIVE-15397
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Sergey Shelukhin
>            Assignee: Sergey Shelukhin
>         Attachments: HIVE-15397.patch
>
>
> Queries like select 1=1 from t group by 1=1 may return rows, based on OneNullRowInputFormat,
even if the source table is empty. For now, add some basic detection of empty tables and turn
this off by default (since we can't know whether a table is empty or not based on there being
some files, without reading them).



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