fixup

planner/core/cache.go

@@ -157,9 +157,7 @@ func (s FieldSlice) CheckTypesCompatibility4PC(tps []*types.FieldType) bool {
 		// string types will show up here, and (2) we don't need flen and decimal to be matched exactly to use plan cache
 		tpEqual := (s[i].GetType() == tps[i].GetType()) ||
 			(s[i].GetType() == mysql.TypeVarchar && tps[i].GetType() == mysql.TypeVarString) ||
-			(s[i].GetType() == mysql.TypeVarString && tps[i].GetType() == mysql.TypeVarchar) ||
-			// TypeNull should be considered the same as other types.
-			(s[i].GetType() == mysql.TypeNull || tps[i].GetType() == mysql.TypeNull)
+			(s[i].GetType() == mysql.TypeVarString && tps[i].GetType() == mysql.TypeVarchar)
 		if !tpEqual || s[i].GetCharset() != tps[i].GetCharset() || s[i].GetCollate() != tps[i].GetCollate() ||
 			(s[i].EvalType() == types.ETInt && mysql.HasUnsignedFlag(s[i].GetFlag()) != mysql.HasUnsignedFlag(tps[i].GetFlag())) {
 			return false