Github user xuanyuanking commented on a diff in the pull request:
https://github.com/apache/spark/pull/19773#discussion_r216122599
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/command/ddl.scala ---
@@ -350,16 +366,11 @@ case class AlterTableChangeColumnCommand(
s"${schema.fieldNames.mkString("[`", "`, `", "`]")}"))
}
- // Add the comment to a column, if comment is empty, return the original
column.
- private def addComment(column: StructField, comment: Option[String]):
StructField = {
- comment.map(column.withComment(_)).getOrElse(column)
- }
-
--- End diff --
Thanks for your question, actually that's also what I'm consider during do
the compatible check. Hive do this column type change work in
[HiveAlterHandler](https://github.com/apache/hive/blob/3287a097e31063cc805ca55c2ca7defffe761b6f/standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/metastore/HiveAlterHandler.java#L175
) and the detailed compatible check is in
[ColumnType](https://github.com/apache/hive/blob/3287a097e31063cc805ca55c2ca7defffe761b6f/standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/metastore/ColumnType.java#L206).
You can see in the ColumnType checking work, it actually use the `canCast`
semantic to judge compatible.
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