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From "Vineet Garg (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (HIVE-21690) Support outer joins with HiveAggregateJoinTransposeRule and turn it on by default
Date Mon, 06 May 2019 23:02:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-21690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16834259#comment-16834259
] 

Vineet Garg edited comment on HIVE-21690 at 5/6/19 11:01 PM:
-------------------------------------------------------------

bq. Why should we consider only aggregate operators? What about other operators?
One of the reason for considering aggregate operator beside join operator is that similar
to join operator it involves (in most of the cases) shuffling data and therefore it adds significant
cost to the overall execution unlike other operators.

bq. Since cost model is pluggable, have you thought about creating a cost model that extends
the join reordering (default) one with cost calculation for the Aggregate operator? You could
use the new cost model when you trigger this rule. In a follow-up, you can study whether using
the same cost model for join reordering makes sense or not, and evaluate the merit of that
change for join reordering on its own.
I really like this suggestion and I think this is better approach. Like you suggested it is
prudent to  evaluate the change in cost model before making it permanent.


was (Author: vgarg):
bq. Why should we consider only aggregate operators? What about other operators?
One of the reason for considering aggregate operator beside join operator is that similar
to join operator it involves (in most of the cases) shuffling data and therefore it adds significant
cost to the overall execution unlike other operators.

bq. Since cost model is pluggable, have you thought about creating a cost model that extends
the join reordering (default) one with cost calculation for the Aggregate operator? You could
use the new cost model when you trigger this rule. In a follow-up, you can study whether using
the same cost model for join reordering makes sense or not, and evaluate the merit of that
change for join reordering on its own.
I really like this suggestion and I think this is better approach. Like you suggested it is
prudent to  evaluate the change in cost model change before making it permanent.

> 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
>         Attachments: HIVE-21690.1.patch
>
>
> 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.
>  
> cc [~jcamachorodriguez] [~ashutoshc] [~gopalv]



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