Fokko commented on a change in pull request #26644: [SPARK-30004][SQL] Allow 
merge UserDefinedType into a native DataType
URL: https://github.com/apache/spark/pull/26644#discussion_r351154695
 
 

 ##########
 File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala
 ##########
 @@ -453,7 +453,23 @@ case class StructType(fields: Array[StructField]) extends 
DataType with Seq[Stru
    * 3. If B doesn't exist in `this`, it's also included in the result schema.
    * 4. Otherwise, `this` and `that` are considered as conflicting schemas and 
an exception would be
    *    thrown.
+   *
+   * Function to merge the two DataTypes that is compatible with the left
+   * and right side. The order is:
+   *
+   * 1. Merge Arrays, where the type of the Arrays should be compatible
+   * 2. Merge Maps, where the type of the Maps should be compatible
+   * 3. Merge Structs, where the struct recursively checked for compatibility
+   * 4. Merge DecimalType, where the scale and precision should be equal
+   * 5. Merge UserDefinedType, where the underlying class is equal
+   * 6. Merge UserDefinedType into DataType, where the underlying DataType is 
equal
+   * 7. Merge DataType, where we check if the DataTypes are compatible
+   * 8. Unable to determine compatibility or incompatible, throw exception
+   *
+   * @throws org.apache.spark.SparkException In case the DataTypes are 
incompatible
+   * @return The compatible DataType that support both left and right
    */
+  @throws(classOf[SparkException])
 
 Review comment:
   Sure, I've removed it for now.

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