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