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From "Maryann Xue (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (PHOENIX-1556) Base hash versus sort merge join decision on cost
Date Mon, 05 Feb 2018 08:32:00 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-1556?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16352053#comment-16352053
] 

Maryann Xue edited comment on PHOENIX-1556 at 2/5/18 8:31 AM:
--------------------------------------------------------------

Could you please review the patch for me, [~jamestaylor]?

The {{CostBasedDecisionIT#testJoinStrategyXXX()}} tests are to verify and demonstrate how
join strategies are chosen in different scenarios.

The calculation of join costs is designed to follow the guidelines below:
 # The build side is applied with more weight than the probe side in calculating the cost
of a hash-join, so the smaller table is guaranteed to be the build side.
 # If the build side exceeds the size limit (QueryServices.MAX_SERVER_CACHE_SIZE_ATTRIB),
the cost of hash-join is infinitely large.
 # The cost of sort-merge-join alone is guaranteed to be smaller than hash-join. But when
sorting is required to do a sort-merge-join, the cost of the added "order-by" will be counted
in the total cost and thus may make the sort-merge-join operation as a whole more expensive.
 # The {{QueryCompiler#compileJoinQuery()}} method will compare the costs of the plans from
all applicable join strategies and chose a local optimal. As a result, with multiple joins
(joins between more than two tables), a mix of join strategies may be chosen as the final
plan. For example, (A hash-join B) sort-merge-join C.


was (Author: maryannxue):
Could you please review the patch for me, [~jamestaylor]?

> Base hash versus sort merge join decision on cost
> -------------------------------------------------
>
>                 Key: PHOENIX-1556
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-1556
>             Project: Phoenix
>          Issue Type: Sub-task
>            Reporter: James Taylor
>            Assignee: Maryann Xue
>            Priority: Major
>              Labels: CostBasedOptimization
>         Attachments: PHOENIX-1556.patch
>
>
> At compile time, we know how many guideposts (i.e. how many bytes) will be scanned for
the RHS table. We should, by default, base the decision of using the hash-join verus many-to-many
join on this information.
> Another criteria (as we've seen in PHOENIX-4508) is whether or not the tables being
joined are already ordered by the join key. In that case, it's better to always use the sort
merge join.



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