skip generalizing type if two types are equal

Author: angelamayxie

enginetest/queries/integration_plans.go

@@ -8730,7 +8730,8 @@ where
 	},
 	{
 		// https://github.com/dolthub/dolt/issues/9024
-		Name: "subqueries should coerce union types",
+		Name:    "subqueries should coerce union types",
+		Dialect: "mysql",
 		SetUpScript: []string{
 			"create table enum_table (i int primary key, e enum('a','b'))",
 			"insert into enum_table values (1,'a'),(2,'b')",

enginetest/queries/script_queries.go

@@ -16,6 +16,7 @@ package types
 
 import (
 	"fmt"
+	"reflect"
 	"strconv"
 	"strings"
 	"time"
@@ -635,7 +636,12 @@ func generalizeNumberTypes(a, b sql.Type) sql.Type {
 // GeneralizeTypes returns the more "general" of two types as defined by
 // https://dev.mysql.com/doc/refman/8.4/en/flow-control-functions.html
 // TODO: Create and handle "Illegal mix of collations" error
+// TODO: Handle extended types, like DoltgresType
 func GeneralizeTypes(a, b sql.Type) sql.Type {
+	if reflect.DeepEqual(a, b) {
+		return a
+	}
+
 	if a == Null {
 		return b
 	}