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https://issues.apache.org/jira/browse/HIVE-21690?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Vineet Garg reassigned HIVE-21690:
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> Support outer joins with HiveAggregateJoinTransposeRule and turn it on by 
> default
> ---------------------------------------------------------------------------------
>
>                 Key: HIVE-21690
>                 URL: https://issues.apache.org/jira/browse/HIVE-21690
>             Project: Hive
>          Issue Type: Improvement
>          Components: Query Planning
>            Reporter: Vineet Garg
>            Assignee: Vineet Garg
>            Priority: Major
>
> 1) This optimization is off by default. We would like to turn on this 
> optimization wherein group by is pushed down to join, in some cases top 
> aggregate is removed but in most of the cases this optimization adds extra 
> aggregate nodes. To measure if those extra aggregates are beneficial or not 
> (they might add extra overhead without reducing rows) cost is computed and 
> compared b/w previous plan and new plan.
> Since Hive's cost model only consider JOIN's cost and discard cost of rest of 
> the nodes, this comparison always favor new plan (since adding aggregate 
> beneath join reduces the total number of rows processed by the join and 
> therefore reduces the join cost). Therefore turning on this optimization with 
> existing cost model is not a good idea.
> One approach to fix this is to localize the cost computation to the rule 
> itself, i.e compute the non-cumulative cost of existing aggregate and join 
> and compare it with new cost of new aggregates, join and top aggregate. 
> Better approach in my opinion would be to fix the cost model and take 
> aggregate cost into account (along with the join). This could affect other 
> queries and can cause performance regression but those will most likely be 
> issues with the planning and should be investigated and fixed.
> 2) This optimization currently only support INNER JOIN. This can be extended 
> to support OUTER joins.



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