Wenchen Fan resolved SPARK-17791.
       Resolution: Fixed
    Fix Version/s: 2.2.0

Issue resolved by pull request 15363

> Join reordering using star schema detection
> -------------------------------------------
>                 Key: SPARK-17791
>                 URL: https://issues.apache.org/jira/browse/SPARK-17791
>             Project: Spark
>          Issue Type: Sub-task
>          Components: SQL
>    Affects Versions: 2.1.0
>            Reporter: Ioana Delaney
>            Assignee: Ioana Delaney
>            Priority: Critical
>             Fix For: 2.2.0
>         Attachments: StarJoinReordering1214.doc
> This JIRA is a sub-task of SPARK-17626.
> The objective is to provide a consistent performance improvement for star 
> schema queries. Star schema consists of one or more fact tables referencing a 
> number of dimension tables. In general, queries against star schema are 
> expected to run fast  because of the established RI constraints among the 
> tables. This design proposes a join reordering based on natural, generally 
> accepted heuristics for star schema queries:
> * Finds the star join with the largest fact table and places it on the 
> driving arm of the left-deep join. This plan avoids large tables on the 
> inner, and thus favors hash joins. 
> * Applies the most selective dimensions early in the plan to reduce the 
> amount of data flow.
> The design description is included in the below attached document.
> \\

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