support change column comment for external hive table

Author: jiangxb1987

sql/core/src/main/scala/org/apache/spark/sql/execution/command/ddl.scala

@@ -314,8 +314,8 @@ case class AlterTableChangeColumnCommand(
     val resolver = sparkSession.sessionState.conf.resolver
     DDLUtils.verifyAlterTableType(catalog, table, isView = false)
 
-    // Find the origin column from schema by column name.
-    val originColumn = findColumnByName(table.schema, columnName, resolver)
+    // Find the origin column from dataSchema by column name.
+    val originColumn = findColumnByName(table.dataSchema, columnName, resolver)
     // Throw an AnalysisException if the column name/dataType is changed.
     if (!columnEqual(originColumn, newColumn, resolver)) {
       throw new AnalysisException(
@@ -324,16 +324,15 @@ case class AlterTableChangeColumnCommand(
           s"'${newColumn.name}' with type '${newColumn.dataType}'")
     }
 
-    val newSchema = table.schema.fields.map { field =>
+    val newDataSchema = table.dataSchema.fields.map { field =>
       if (field.name == originColumn.name) {
         // Create a new column from the origin column with the new comment.
         addComment(field, newColumn.getComment)
       } else {
         field
       }
     }
-    val newTable = table.copy(schema = StructType(newSchema))
-    catalog.alterTable(newTable)
+    catalog.alterTableDataSchema(tableName, StructType(newDataSchema))
 
     Seq.empty[Row]
   }

sql/core/src/test/scala/org/apache/spark/sql/execution/command/DDLSuite.scala

@@ -1597,6 +1597,7 @@ abstract class DDLSuite extends QueryTest with SQLTestUtils {
     // Ensure that change column will preserve other metadata fields.
     sql("ALTER TABLE dbx.tab1 CHANGE COLUMN col1 col1 INT COMMENT 'this is col1'")
     assert(getMetadata("col1").getString("key") == "value")
+    assert(getMetadata("col1").getString("comment") == "this is col1")
   }
 
   test("drop build-in function") {