diff --git a/src/sql/engine/expr/ob_expr_cast.cpp b/src/sql/engine/expr/ob_expr_cast.cpp
index 2120d4588..cc53d46fb 100644
--- a/src/sql/engine/expr/ob_expr_cast.cpp
+++ b/src/sql/engine/expr/ob_expr_cast.cpp
@@ -634,6 +634,10 @@ int ObExprCast::adjust_udt_cast_type(const ObExprResType &src_type, ObExprResTyp
         LOG_WARN("cast unsupported sql udt type to pl udt type", K(ret), K(src_type), K(dst_type));
       }
     }
+  } else if (src_type.is_null()) {
+    if (dst_type.get_type() == ObUserDefinedSQLType) {
+      dst_type.set_sql_udt(ObXMLSqlType);
+    }
   }
   return ret;
 }
diff --git a/src/sql/engine/expr/ob_expr_result_type_util.cpp b/src/sql/engine/expr/ob_expr_result_type_util.cpp
index 5dc044840..9ca811f8c 100644
--- a/src/sql/engine/expr/ob_expr_result_type_util.cpp
+++ b/src/sql/engine/expr/ob_expr_result_type_util.cpp
@@ -760,6 +760,14 @@ int ObExprResultTypeUtil::deduce_max_string_length_oracle(const ObDataTypeCastPa
           length /= ObCharset::get_charset(target_type.get_collation_type())->mbminlen;
         }
       }
+    } else if (orig_type.is_user_defined_sql_type() || orig_type.is_ext()) {
+      // udt types like xml can cast to string, the accuracy in pl extend is used for udt id
+      if (LS_CHAR == length_semantics) {
+        int64_t mbminlen = ObCharset::get_charset(target_type.get_collation_type())->mbminlen;
+        length = OB_MAX_VARCHAR_LENGTH_KEY / mbminlen;
+      } else {
+        length = OB_MAX_VARCHAR_LENGTH_KEY; // issue 49536718: CREATE INDEX index ON table (UPPER(c1));
+      }
     } else {
       int64_t ascii_bytes = 0;
       if (orig_type.is_null()) {