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https://issues.apache.org/jira/browse/HIVE-14866?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Work on HIVE-14866 started by Jesus Camacho Rodriguez.
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> Set hive.limit.optimize.enable to true by default
> -------------------------------------------------
>
> Key: HIVE-14866
> URL: https://issues.apache.org/jira/browse/HIVE-14866
> Project: Hive
> Issue Type: Improvement
> Affects Versions: 2.1.0
> Reporter: Jesus Camacho Rodriguez
> Assignee: Jesus Camacho Rodriguez
>
> Currently, we set up the global limit for the query in two different places
> through two different variables: SemanticAnalyzer and through an optimization
> rule GlobalLimitOptimizer (the latest is off by default).
> This leads to several problems that I have observed:
> - Global limit might not be set for very simple queries, e.g., if the query
> does not contain a RS). GlobalLimitOptimizer would set the limit in this
> case, but as stated above, it is off by default.
> - Some other optimizations are not checking both variables, thus missing
> opportunities.
> - The variable set by SemanticAnalyzer does not take into account offset of
> the query, which I think might lead to incorrect results if FetchOptimizer
> kicks in (not verified yet). GlobalLimitOptimizer does take into account
> offset of query.
> This issue is to set hive.limit.optimize.enable to _true_ by default, i.e.,
> use GlobalLimitOptimizer, and thus getting rid of the variable set by
> SemanticAnalyzer. Maybe there are some gaps (cases covered by
> SemanticAnalyzer alternative and not covered by GlobalLimitOptimizer) that we
> will need to work on.
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