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https://issues.apache.org/jira/browse/HIVE-27142?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated HIVE-27142:
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Labels: pull-request-available (was: )
> Map Join not working as expected when joining non-native tables with native
> tables
> -----------------------------------------------------------------------------------
>
> Key: HIVE-27142
> URL: https://issues.apache.org/jira/browse/HIVE-27142
> Project: Hive
> Issue Type: Bug
> Affects Versions: All Versions
> Reporter: Syed Shameerur Rahman
> Assignee: Syed Shameerur Rahman
> Priority: Major
> Labels: pull-request-available
> Fix For: 4.0.0
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> *1. Issue :*
> When *_hive.auto.convert.join=true_* and if the underlying query is trying to
> join a large non-native hive table with a small native hive table, The map
> join is happening in the wrong side i.e on the map task which process the
> small native hive table and it can lead to OOM when the non-native table is
> really large and only few map tasks are spawned to scan the small native hive
> tables.
>
> *2. Why is this happening ?*
> This happens due to improper stats collection/computation of non native hive
> tables. Since the non-native hive tables are actually stored in a different
> location which Hive does not know of and only a temporary path which is
> visible to Hive while creating a non native table does not store the actual
> data, The stats collection logic tend to under estimate the data/rows and
> hence causes the map join to happen in the wrong side.
>
> *3. Potential Solutions*
> 3.1 Turn off *_hive.auto.convert.join=false._* This can have a negative
> impact of the query if the same query is trying to do multiple joins i.e
> one join with non-native tables and other join where both the tables are
> native.
> 3.2 Compute stats for non-native table by firing the ANALYZE TABLE <>
> command before joining native and non-native commands. The user may or may
> not choose to do it.
> 3.3 Don't collect/estimate stats for non-native hive tables by default
> (Preferred solution)
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