cloud-fan commented on a change in pull request #27991: [SPARK-31227][SQL]
Non-nullable null type in complex types should not coerce to nullable type
URL: https://github.com/apache/spark/pull/27991#discussion_r397053082
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File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
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@@ -210,8 +206,15 @@ object Cast {
case _ => false // overflow
}
+ /**
+ * Returns `true` iff it should change the nullability of this type in map
type,
+ * array type, expressions such as cast, etc.
+ *
+ * Note that you should take the nullability context into account.
+ * For example, it should not force to nullable type when null type in array
+ * type is non-nullable which means an empty array of null type.
+ */
Review comment:
How about we define `Cast.nullable` as `child.nullable ||
Cast.forceNullable(child.dataType, dataType)`?
Then we can say that: when calling this method, the `from` type is not
nullable. If `from == NullType`, it means empty array or map, and it does not
force the result to be nullable.
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