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https://issues.apache.org/jira/browse/HIVE-3784?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13563749#comment-13563749
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Ashutosh Chauhan commented on HIVE-3784:
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I am still reviewing the code. I have added some initial comments on
phabricator. But, at this point my major concern is following:
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. In all three cases
there will be performance impact for end-users. Ideally, I would like to see
all these cases handled before this get in, though I am particularly concerned
about c) common join after mapjoin, since that looks relatively more common
use-case than a) or b). Can you take a look and see how hard it will be do it?
> 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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