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From "Namit Jain (JIRA)" <>
Subject [jira] [Commented] (HIVE-3784) de-emphasize mapjoin hint
Date Tue, 22 Jan 2013 06:36:13 GMT


Namit Jain commented on HIVE-3784:

I was thinking of adding a size parameter. If n-1 tables are below that size (for a n-way
join), the joinTask should be converted to a mapJoin task
(map-only) instead of a conditional task. We would need a further optimization step to merge
2 map-only tasks to a single map-only task.

[~navis], what do you think ? Can you think of a better idea ?
> de-emphasize mapjoin hint
> -------------------------
>                 Key: HIVE-3784
>                 URL:
>             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.3784.6.patch
> 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 to true
> and not specifying the hint. This will simplify the code a lot.
> What does everyone think ?

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