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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-12334) Partition Map Join
Date Wed, 04 Nov 2015 21:19:27 GMT

    [ https://issues.apache.org/jira/browse/HIVE-12334?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14990441#comment-14990441
] 

Gopal V commented on HIVE-12334:
--------------------------------

This is an intriguing idea, with the layout partitioning that can be expanded as standard
SQL unions?

{code}
SELECT /*+ MAPJOIN(b) */ a.key, a.value
FROM a JOIN b ON a.key = b.key and a.key
{code}

becomes 

{code}
SELECT a.key, a.value from a,b where a.key = b.key and a.key = "20151208" and b.key = "20151208"
UNION ALL
SELECT a.key, a.value from a,b where a.key = b.key and a.key = "20151209" and b.key = "20151309"
...
{code}

for the set intersection of a.key & b.key?

> Partition Map Join
> ------------------
>
>                 Key: HIVE-12334
>                 URL: https://issues.apache.org/jira/browse/HIVE-12334
>             Project: Hive
>          Issue Type: Improvement
>          Components: Logical Optimizer, Physical Optimizer, SQL
>    Affects Versions: 0.13.0, 0.14.0, 0.13.1, 1.0.0, 1.1.0
>            Reporter: Maciek Kocon
>              Labels: gsoc2015
>
> Logically and functionally bucketing and partitioning are quite similar - both provide
mechanism to segregate and separate the table's data based on its content. Thanks to that
significant further optimisations like [partition] PRUNING or [bucket] MAP JOIN are possible.
> The difference seems to be imposed by design where the PARTITIONing is open/explicit
while BUCKETing is discrete/implicit.
> Partitioning seems to be very common if not a standard feature in all current RDBMS while
BUCKETING seems to be HIVE specific only.
> In a way BUCKETING could be also called by "hashing" or simply "IMPLICIT PARTITIONING".
> Regardless of the fact that these two are recognised as two separate features available
in Hive there should be nothing to prevent leveraging same existing query/join optimisations
across the two.
> PARTITION MAPJOIN
> Use the same type of optimization as in BUCKETED MAP JOIN for PARTITIONED tables.
> The partition map join could be performed if the tables being joined are partitioned
on the join columns.
> If table A has set partitioning on KEY column and table B is partitioned on KEY column,
the following join
> SELECT /*+ MAPJOIN(b) */ a.key, a.value
> FROM a JOIN b ON a.key = b.key
> can be done on the mapper only. Instead of fetching B completely for each mapper of A,
only the required partitions are fetched. For the query above, the mapper processing partition
key='20151208' for A will only fetch partition for key='20151208' of B.



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