[jira] [Commented] (SPARK-17791) Join reordering using star schema detection

["Apache Spark (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-17791?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15550178#comment-15550178 ] 

Apache Spark commented on SPARK-17791:
--------------------------------------

User 'ioana-delaney' has created a pull request for this issue:
https://github.com/apache/spark/pull/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
>            Priority: Critical
>         Attachments: StarJoinReordering1005.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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