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https://issues.apache.org/jira/browse/SPARK-8653?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14609609#comment-14609609
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Cheng Hao commented on SPARK-8653:
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For most of the Mathematical expressions, we can get the generic rule for the
data type casting implicitly. But seems arbitrary rules for the others. For
example, there are lots of custom rules for `Coalesce`, Conditional
expressions, (CaseWhen, In, If etc.) already, we probably don't want to keep
growing the rules in the `HiveTypeCoercion.scala`.
So probably it's a good a idea to expose the data type casting to expression
itself, and the UDAF, UDTF will also be benifit from that. Additionally, if we
want to make the `Expression` as API for extension in the future, it will be a
block issue if the data type casting is only allowed to be done in
`HiveTypeCoercion.scala`.
> Add constraint for Children expression for data type
> ----------------------------------------------------
>
> Key: SPARK-8653
> URL: https://issues.apache.org/jira/browse/SPARK-8653
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Reporter: Cheng Hao
> Assignee: Reynold Xin
>
> Currently, we have trait in Expression like `ExpectsInputTypes` and also the
> `checkInputDataTypes`, but can not convert the children expressions
> automatically, except we write the new rules in the `HiveTypeCoercion`.
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