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https://issues.apache.org/jira/browse/HIVE-28734?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17924014#comment-17924014
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Stamatis Zampetakis commented on HIVE-28734:
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I just logged HIVE-27841 to mark the deprecation of hive.cbo.fallback.strategy
property. I am not against adding fixes to CONSERVATIVE option but since the
whole property is going to disappear it probably not worth investing much
effort on this domain. I would encourage contributors and interviewers to focus
and improve other areas of the code rather than working on deprecated toggles.
> Part 2 Skip CBO fallback when EXCEPT/INTERSECT/QUALIFY are used
> ---------------------------------------------------------------
>
> Key: HIVE-28734
> URL: https://issues.apache.org/jira/browse/HIVE-28734
> Project: Hive
> Issue Type: Improvement
> Components: CBO
> Affects Versions: 4.0.0
> Reporter: Konstantin Bereznyakov
> Assignee: Konstantin Bereznyakov
> Priority: Major
> Labels: pull-request-available
> Fix For: 4.1.0
>
>
> Cloning the original issue (HIVE-28193) due to finding more examples of
> misleading exceptions.
> For example, a query like
>
> {{SELECT * FROM table1 a CROSS JOIN table1 b EXCEPT SELECT * FROM table1 a
> CROSS JOIN table1 b}}
> fails with
> {{ql.Driver: FAILED: SemanticException EXCEPT and INTERSECT operations are
> only supported with Cost Based Optimizations enabled. Please set
> 'hive.cbo.enable' to true! org.apache.hadoop.hive.ql.parse.SemanticException:
> EXCEPT and INTERSECT operations are only supported with Cost Based
> Optimizations enabled. Please set 'hive.cbo.enable' to true!}}
>
> *Original from HIVE-28193 below:*
> With `hive.cbo.fallback.strategy=CONSERVATIVE`, Hive retries to compile an
> SQL without CBO when a compile error happens. We sometimes see an unkind
> error message when EXCEPT, INTERSECT, OR QUALIFY are used. The fallback
> non-CBO mode doesn't support those operators, so Hive asked us to enable CBO.
> {code:java}
> 0: jdbc:hive2://hive-hiveserver2:10000/defaul> select cast(0 as bigint) = '1'
> EXCEPT select cast(1 as bigint) = '1';
> Error: Error while compiling statement: FAILED: SemanticException EXCEPT and
> INTERSECT operations are only supported with Cost Based Optimizations
> enabled. Please set 'hive.cbo.enable' to true! (state=42000,code=40000)
> 0: jdbc:hive2://hive-hiveserver2:10000/defaul> set
> hive.cbo.fallback.strategy=NEVER;
> No rows affected (0.035 seconds)
> 0: jdbc:hive2://hive-hiveserver2:10000/defaul> select cast(0 as bigint) = '1'
> EXCEPT select cast(1 as bigint) = '1';
> Error: Error while compiling statement: FAILED: SemanticException Line 0:-1
> Wrong arguments ''1'': Unsafe compares between different types are disabled
> for safety reasons. If you know what you are doing, please set
> hive.strict.checks.type.safety to false and make sure that hive.mapred.mode
> is not set to 'strict' to proceed. Note that you may get errors or incorrect
> results if you make a mistake while using some of the unsafe features.
> (state=42000,code=40000) {code}
> It is inconvenient, especially when the CBO mode fails because of
> `hive.strict.checks.*`, because we can't identify the true reason why the
> query failed.
> We plan to disable the fallback behavior or deprecate the non-CBO mode.
> However, we sometimes need to rely on the features at this point.
> * https://issues.apache.org/jira/browse/HIVE-27831
> * https://issues.apache.org/jira/browse/HIVE-27830
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