Vineet Garg commented on HIVE-17308:

First patch introduces different cardinality estimation if there are multiple 
join keys (since hive doesn't have any way to figure out if there is 
correlation we always assume correlation).

> Improvement in join cardinality estimation
> ------------------------------------------
>                 Key: HIVE-17308
>                 URL: https://issues.apache.org/jira/browse/HIVE-17308
>             Project: Hive
>          Issue Type: Improvement
>          Components: Query Planning
>            Reporter: Vineet Garg
>            Assignee: Vineet Garg
>         Attachments: HIVE-17308.1.patch
> Currently during logical planning join cardinality is estimated assuming no 
> correlation among join keys (This estimation is done using exponential 
> backoff). Physical planning on the other hand consider correlation for multi 
> keys and uses different estimation. We should consider correlation during 
> logical planning as well.

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