viirya commented on a change in pull request #34038:
URL: https://github.com/apache/spark/pull/34038#discussion_r714490029



##########
File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CheckAnalysis.scala
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@@ -401,15 +401,30 @@ trait CheckAnalysis extends PredicateHelper with 
LookupCatalog {
                     |the ${ordinalNumber(ti + 1)} table has 
${child.output.length} columns
                   """.stripMargin.replace("\n", " ").trim())
               }
+              val isUnion = operator.isInstanceOf[Union]
+              val dataTypesAreCompatibleFn = if (isUnion) {
+                // `TypeCoercion` takes care of type coercion already. If any 
columns or nested
+                // columns are not compatible, we detect it here and throw 
analysis exception.
+                val typeChecker = (dt1: DataType, dt2: DataType) => {
+                  !TypeCoercion.findWiderTypeForTwo(dt1.asNullable, 
dt2.asNullable).isEmpty

Review comment:
       Oh, I spent a little time to recall why I keep original check logic.
   
   It is because if `TypeCoercion` fails to find compatible types for all 
columns, it won't add cast for all. It is all or nothing logic here.
   
   So if we only check `dt1 == dt2` here, we compare the original data types 
even some of them are compatible.
   
   `AnalysisErrorSuite` has one example. One relation has `short, string, 
double, decimal`, another one has `string, string, string, map`.
   
   The first three columns are compatible, only the fourth isn't. So 
`TypeCoercion` fails to add casts for all.
   
   If we compare `dt1 == dt2`, the error will be like "short is not compatible 
with string". But currently we get like "decimal is not compatible with map".
   




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