MDEV-7207 - ALTER VIEW does not change ALGORITM

Fixed that ALTER VIEW ALGORITHM=UNDEFINED behaved as if algorithm was not
specified.

mysql-test/r/view.result

@@ -5411,6 +5411,24 @@ create view v2 as select t2.* from (t2 left join v1 using (id));
 update t3 left join v2 using (id) set flag=flag+1;
 drop view v2, v1;
 drop table t1, t2, t3;
+#
+# MDEV-7207 - ALTER VIEW does not change ALGORITM
+#
+create table t1 (a int, b int);
+create algorithm=temptable view v2 (c) as select b+1 from t1;
+show create view v2;
+View	Create View	character_set_client	collation_connection
+v2	CREATE ALGORITHM=TEMPTABLE DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v2` AS select (`t1`.`b` + 1) AS `c` from `t1`	latin1	latin1_swedish_ci
+alter algorithm=undefined view v2 (c) as select b+1 from t1;
+show create view v2;
+View	Create View	character_set_client	collation_connection
+v2	CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v2` AS select (`t1`.`b` + 1) AS `c` from `t1`	latin1	latin1_swedish_ci
+alter algorithm=merge view v2 (c) as select b+1 from t1;
+show create view v2;
+View	Create View	character_set_client	collation_connection
+v2	CREATE ALGORITHM=MERGE DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v2` AS select (`t1`.`b` + 1) AS `c` from `t1`	latin1	latin1_swedish_ci
+drop view v2;
+drop table t1;
 # -----------------------------------------------------------------
 # -- End of 5.5 tests.
 # -----------------------------------------------------------------

mysql-test/t/view.test

@@ -5367,6 +5367,19 @@ update t3 left join v2 using (id) set flag=flag+1;
 drop view v2, v1;
 drop table t1, t2, t3;
 
+--echo #
+--echo # MDEV-7207 - ALTER VIEW does not change ALGORITM
+--echo #
+create table t1 (a int, b int);
+create algorithm=temptable view v2 (c) as select b+1 from t1;
+show create view v2;
+alter algorithm=undefined view v2 (c) as select b+1 from t1;
+show create view v2;
+alter algorithm=merge view v2 (c) as select b+1 from t1;
+show create view v2;
+drop view v2;
+drop table t1;
+
 --echo # -----------------------------------------------------------------
 --echo # -- End of 5.5 tests.
 --echo # -----------------------------------------------------------------

sql/sql_view.cc

@@ -228,7 +228,7 @@ fill_defined_view_parts (THD *thd, TABLE_LIST *view)
     view->definer.user= decoy.definer.user;
     lex->definer= &view->definer;
   }
-  if (lex->create_view_algorithm == DTYPE_ALGORITHM_UNDEFINED)
+  if (lex->create_view_algorithm == VIEW_ALGORITHM_INHERIT)
     lex->create_view_algorithm= (uint8) decoy.algorithm;
   if (lex->create_view_suid == VIEW_SUID_DEFAULT)
     lex->create_view_suid= decoy.view_suid ? 

sql/sql_yacc.yy

@@ -6552,7 +6552,7 @@ alter:
               my_error(ER_SP_BADSTATEMENT, MYF(0), "ALTER VIEW");
               MYSQL_YYABORT;
             }
-            lex->create_view_algorithm= DTYPE_ALGORITHM_UNDEFINED;
+            lex->create_view_algorithm= VIEW_ALGORITHM_INHERIT;
             lex->create_view_mode= VIEW_ALTER;
           }
           view_tail

sql/table.h

@@ -1446,6 +1446,8 @@ typedef struct st_schema_table
 #define DT_PHASES_MATERIALIZE (DT_COMMON | DT_MATERIALIZE)
 
 #define VIEW_ALGORITHM_UNDEFINED 0
+/* Special value for ALTER VIEW: inherit original algorithm. */
+#define VIEW_ALGORITHM_INHERIT   DTYPE_VIEW
 #define VIEW_ALGORITHM_MERGE    (DTYPE_VIEW | DTYPE_MERGE)
 #define VIEW_ALGORITHM_TMPTABLE (DTYPE_VIEW | DTYPE_MATERIALIZE)