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https://issues.apache.org/jira/browse/HIVE-13069?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15174108#comment-15174108
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Jesus Camacho Rodriguez commented on HIVE-13069:
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[~ashutoshc], that was my first idea indeed: to ensure leftAlias is not null.
However, leftAlias in QBJoinTree is only set if there is a join condition.
In fact, I have been checking the code again, and I think it is safe to merge
checking that 1) leftAlias is null, and 2) the number of conditions in node and
target node are the same i.e. zero.
I am uploading a new patch that will add new test cases.
> Enable cartesian product merging
> --------------------------------
>
> Key: HIVE-13069
> URL: https://issues.apache.org/jira/browse/HIVE-13069
> Project: Hive
> Issue Type: Improvement
> Components: Parser
> Reporter: Jesus Camacho Rodriguez
> Assignee: Jesus Camacho Rodriguez
> Attachments: HIVE-13069.patch
>
>
> Currently we can merge 2-way joins into n-way joins when the joins are
> executed over the same column.
> In turn, CBO might produce plans containing cartesian products if the join
> columns are constant values; after HIVE-12543 went in, this is rather common,
> as those constant columns are correctly pruned. However, currently we do not
> merge a cartesian product with two inputs into a cartesian product with
> multiple inputs, which could result in performance loss.
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