Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/21169#discussion_r184589060
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala
---
@@ -782,6 +782,22 @@ object TypeCoercion {
// Skip nodes who's children have not been resolved yet.
case e if !e.childrenResolved => e
+ // Special rules for `to/from_utc_timestamp`.
`to/from_utc_timestamp` assumes its input is
+ // in UTC timezone, and if input is string, it should not contain
timezone.
+ // TODO: We should move the type coercion logic to expressions
instead of a central
+ // place to put all the rules.
+ case e: FromUTCTimestamp if e.left.dataType == StringType =>
--- End diff --
Catalyst suggests rules in the same batch is order insensitive, since here
this rule must be run before implicit type cast, we'd better put them in the
same rule to guarantee the order.
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