[jira] [Commented] (HIVE-2845) Add support for index joins in Hive

["Mahsa Mofidpoor (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-2845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13405956#comment-13405956 ] 

Mahsa Mofidpoor commented on HIVE-2845:
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When where clause makes use of indexes, the query is re-written to process the index table, rather than the base one. The predicate (lets' assume a single condition) produces one single value to lookup in the index table.
1-In case of a join, for each row in table B a lookup should be done, does it mean that for each row query should be re-written? This would lead to so many temp files.    
2- Lets's say the lookup is done, how the association between the result from lookup and the original base table is maintained? Finally we have to access the base table for extracting the desired columns.

  
                
> Add support for index joins in Hive
> -----------------------------------
>
>                 Key: HIVE-2845
>                 URL: https://issues.apache.org/jira/browse/HIVE-2845
>             Project: Hive
>          Issue Type: New Feature
>          Components: Indexing, Query Processor
>            Reporter: Namit Jain
>              Labels: gsoc, gsoc2012, indexing, joins, performance
>
> Hive supports indexes, which are used for filters currently.
> It would be very useful to add support for index-based joins in Hive.
> If 2 tables A and B are being joined, and an index exists on the join key of A,
> B can be scanned (by the mappers), and for each row in B, a lookup for the corresponding row in A can be performed.
> This can be very useful for some usecases.

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