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https://issues.apache.org/jira/browse/HIVE-3784?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13537479#comment-13537479
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Vinod Kumar Vavilapalli commented on HIVE-3784:
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Was trying to play with the patch, and my earlier concern resurfaced.
bq. With different join keys, it needs some work to merge into a single MR
anyway - that work is independent of this change.
That isn't true. Even today, I am able to get hive to automatically merge
multi-way map-join with different join keys into a single map-only job. With
this patch, we are losing that functionality. For e.g., the following runs as a
single Map only job:
{noformat}
select /*+MAPJOIN(smallTableTwo)*/ idOne, idTwo, value FROM
( select /*+MAPJOIN(smallTableOne)*/ idOne, idTwo, value FROM
bigTable
JOIN
smallTableOne on (bigTable.idOne = smallTableOne.idOne)
) firstjoin
JOIN
smallTableTwo on (firstjoin.idTwo = smallTableTwo.idTwo)
{noformat}
> de-emphasize mapjoin hint
> -------------------------
>
> Key: HIVE-3784
> URL: https://issues.apache.org/jira/browse/HIVE-3784
> Project: Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Namit Jain
> Assignee: Namit Jain
> Attachments: hive.3784.1.patch, hive.3784.2.patch, hive.3784.3.patch,
> hive.3784.4.patch, hive.3784.5.patch
>
>
> hive.auto.convert.join has been around for a long time, and is pretty stable.
> When mapjoin hint was created, the above parameter did not exist.
> The only reason for the user to specify a mapjoin currently is if they want
> it to be converted to a bucketed-mapjoin or a sort-merge bucketed mapjoin.
> Eventually, that should also go away, but that may take some time to
> stabilize.
> There are many rules in SemanticAnalyzer to handle the following trees:
> ReduceSink -> MapJoin
> Union -> MapJoin
> MapJoin -> MapJoin
> This should not be supported anymore. In any of the above scenarios, the
> user can get the mapjoin behavior by setting hive.auto.convert.join to true
> and not specifying the hint. This will simplify the code a lot.
> What does everyone think ?
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