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https://issues.apache.org/jira/browse/HIVE-1721?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927545#action_12927545
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Namit Jain commented on HIVE-1721:
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T2 does not fit in memory completely.
We create a bloom filter for T2, which fits in memory - the assumption here is
that by filtering out a lot of rows from T1, we
are reducing the number of rows that go to the reducer substantially, which
helps the join performance
> use bloom filters to improve the performance of joins
> -----------------------------------------------------
>
> Key: HIVE-1721
> URL: https://issues.apache.org/jira/browse/HIVE-1721
> Project: Hive
> Issue Type: New Feature
> Components: Query Processor
> Reporter: Namit Jain
> Assignee: Siying Dong
>
> In case of map-joins, it is likely that the big table will not find many
> matching rows from the small table.
> Currently, we perform a hash-map lookup for every row in the big table, which
> can be pretty expensive.
> It might be useful to try out a bloom-filter containing all the elements in
> the small table.
> Each element from the big table is first searched in the bloom filter, and
> only in case of a positive match,
> the small table hash table is explored.
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