Github user ssimeonov commented on a diff in the pull request:
https://github.com/apache/spark/pull/21840#discussion_r204250360
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/Column.scala ---
@@ -1234,6 +1234,8 @@ class Column(val expr: Expression) extends Logging {
*/
def over(): Column = over(Window.spec)
+ def copy(field: String, value: Column): Column =
withExpr(StructCopy(expr, field, value.expr))
--- End diff --
> Can .cast() be expressed via the .copy() method if the former one will
support add/delete/update operations?
Are you suggesting `Dataset` adds `copy()` as opposed to `cast()`?
Otherwise, certainly, `Dataset.cast()` can be implemented with
`Dataset.select()` but it's not easy even in the simple cases and becomes
noticeably more complicated when you consider correctly setting nullability &
metadata in the edge cases
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