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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-1721) use bloom filters to improve the performance of joins
Date Tue, 02 Nov 2010 18:46:25 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1721?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927538#action_12927538
] 

Namit Jain commented on HIVE-1721:
----------------------------------

It works very well if one of the tables (T1) is much larger than the other table (T2).
T2 is not small enough to be a candidate for map-join.

Then, via the bloom filter, we can filter out most of the rows of T1

> 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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