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https://issues.apache.org/jira/browse/SPARK-19915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15906018#comment-15906018
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Apache Spark commented on SPARK-19915:
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User 'wzhfy' has created a pull request for this issue:
https://github.com/apache/spark/pull/17240
> Improve join reorder: simplify cost evaluation, postpone column pruning,
> exclude cartesian product
> --------------------------------------------------------------------------------------------------
>
> Key: SPARK-19915
> URL: https://issues.apache.org/jira/browse/SPARK-19915
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Affects Versions: 2.2.0
> Reporter: Zhenhua Wang
>
> 1. Usually cardinality is more important than size, we can simplify cost
> evaluation by using only cardinality. Note that this also enables us to not
> care about column pruing during reordering. Because otherwise, project will
> influence the output size of intermediate joins.
> 2. Do column pruning during reordering is troublesome. Given the first
> change, we can do it right after reordering, then logics for adding projects
> on intermediate joins can be removed. This makes the code simpler and more
> reliable.
> 3. Exclude cartesian products in the "memo". This significantly reduces the
> search space and memory overhead of memo. Otherwise every combination of
> items will exist in the memo. We can find those unjoinable items after
> reordering is finished and put them at the end.
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