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## What changes were proposed in this pull request? The `RemoveRedundantAlias` rule can change the output attributes (the expression id's to be precise) of a query by eliminating the redundant alias producing them. This is no problem for a regular query, but can cause problems for correlated subqueries: The attributes produced by the subquery are used in the parent plan; changing them will break the parent plan. This PR fixes this by wrapping a subquery in a `Subquery` top level node when it gets optimized. The `RemoveRedundantAlias` rule now recognizes `Subquery` and makes sure that the output attributes of the `Subquery` node are retained. ## How was this patch tested? Added a test case to `RemoveRedundantAliasAndProjectSuite` and added a regression test to `SubquerySuite`. Author: Herman van Hovell <hvanhovell@databricks.com> Closes #17278 from hvanhovell/SPARK-19933. |
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