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From Ádám Szita (Jira) <j...@apache.org>
Subject [jira] [Commented] (HIVE-22513) Constant propagation of casted column in filter ops can cause incorrect results
Date Wed, 20 Nov 2019 12:47:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-22513?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16978391#comment-16978391
] 

Ádám Szita commented on HIVE-22513:
-----------------------------------

Committed to master.

Thanks for reviewing Peter and Zoltan.

Note: Before committing I've made the small amend of omitting {{--! qt:dataset:src}} as per
Peter's comment. (I successfully re-ran the test locally.)

> Constant propagation of casted column in filter ops can cause incorrect results
> -------------------------------------------------------------------------------
>
>                 Key: HIVE-22513
>                 URL: https://issues.apache.org/jira/browse/HIVE-22513
>             Project: Hive
>          Issue Type: Bug
>          Components: Query Planning
>            Reporter: Ádám Szita
>            Assignee: Ádám Szita
>            Priority: Major
>         Attachments: HIVE-22513.0.patch, HIVE-22513.1.patch
>
>
> This issue happens if CBO is disabled.
> We should not be propagating constants if the corresponding ExprNodeColumnDesc instance
is wrapped inside a CAST operator as casting might truncate information from the original
column.
> This can happen if we're using CAST in a WHERE clause, which will cause the projected
columns to be replaced in a SELECT operator. Their new value will be the result of casting
which could be a different value compared to that in the original column:
> {code:java}
> set hive.cbo.enable=false;
> set hive.fetch.task.conversion=more; --just for testing convenience
> create table testtb (id string);
> insert into testtb values('2019-11-05 01:01:11');
> select id, CAST(id AS VARCHAR(10)) from testtb where CAST(id AS VARCHAR(9)) = '2019-11-0';
> +------------+------------+
> |     id     |    _c1     |
> +------------+------------+
> | 2019-11-0  | 2019-11-0  |
> +------------+------------+
> 1 row selected (0.168 seconds)
> -- VS expected: 2019-11-05 01:01:11 | 2019-11-05 {code}
> As to what types of casting (from and where types) cause information loss it's hard to
properly keep track of, and I don't think it should be taken into consideration when deciding
whether or not to propagate a constant. Rather than adding a big and potentially convoluted
and fragile check for this, I propose to prevent constant mappings to be spawned out of CASTed
columns.



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