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https://issues.apache.org/jira/browse/HIVE-3784?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13563824#comment-13563824
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Namit Jain commented on HIVE-3784:
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bq. Reading from code it feels like that its not possible to have a) union
before mapjoin b) union after mapjoin c) common join after mapjoin.
Union before/after mapjoin hint will not be supported -- That is not true
completely. Without a map-join hint, joins before/after union
will automatically get converted to a map-join. There is nothing which was
supported before and will not be in future - it will work without
the hint.
bq. common join after mapjoin
Same thing. The only small case if for sort-merge joins, which I added recently
in HIVE-3633. This will also be made automatic soon.
I will address your other comments.
> 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.3784.10.patch, hive.3784.11.patch,
> hive.3784.12.patch, hive.3784.13.patch, hive.3784.14.patch,
> hive.3784.15.patch, hive.3784.16.patch, hive.3784.1.patch, hive.3784.2.patch,
> hive.3784.3.patch, hive.3784.4.patch, hive.3784.5.patch, hive.3784.6.patch,
> hive.3784.7.patch, hive.3784.8.patch, hive.3784.9.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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