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https://issues.apache.org/jira/browse/SPARK-28889?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Hyukjin Kwon resolved SPARK-28889.
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Resolution: Later
> Allow UDTs to define custom casting behavior
> --------------------------------------------
>
> Key: SPARK-28889
> URL: https://issues.apache.org/jira/browse/SPARK-28889
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.4.3
> Reporter: Zachary S Ennenga
> Priority: Minor
>
> Looking at `org.apache.spark.sql.catalyst.expressions.Cast`, UDTs do not
> support any sort of casting except for identity casts, IE:
> {code:java}
> case (udt1: UserDefinedType[_], udt2: UserDefinedType[_]) if udt1.userClass
> == udt2.userClass =>
> true
> {code}
> I propose we add an additional piece of functionality here to allow UDTs to
> define their own canCast and cast functions to allow users to define their
> own cast mechanisms.
> An example of how this might look:
> {code:java}
> case (fromType, toType: UserDefinedType[_]) =>
> toType.canCast(fromType) // Returns boolean
> {code}
> {code:java}
> case (fromType, toType: UserDefinedType[_]) =>
> toType.cast(fromType) // Returns Casting function
> {code}
> The UDT base class would contain a default implementation that replicates
> current behavior (IE no casting).
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