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https://issues.apache.org/jira/browse/SPARK-12616?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15080753#comment-15080753
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Xiao Li commented on SPARK-12616:
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I am working on it. Thanks!
> Union logical plan should support arbitrary number of children (rather than
> binary)
> -----------------------------------------------------------------------------------
>
> Key: SPARK-12616
> URL: https://issues.apache.org/jira/browse/SPARK-12616
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Reporter: Reynold Xin
>
> Union logical plan is a binary node. However, a typical use case for union is
> to union a very large number of input sources (DataFrames, RDDs, or files).
> It is not uncommon to union hundreds of thousands of files. In this case, our
> optimizer can become very slow due to the large number of logical unions. We
> should change the Union logical plan to support an arbitrary number of
> children, and add a single rule in the optimizer (or analyzer?) to collapse
> all adjacent Unions into one.
> Note that this problem doesn't exist in physical plan, because the physical
> Union already supports arbitrary number of children.
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