MDEV-22180 Planner opens unnecessary tables when updated table is referenced by foreign keys

only MDL-prelock but do not open FK child tables for read-only (RESTRICT)
FK actions.

Tables still needs to be opened for CASCADE actions, see 9180e866

mysql-test/suite/innodb/r/foreign-keys.result

@@ -124,3 +124,24 @@ SET debug_sync='reset';
 SHOW OPEN TABLES FROM test;
 Database	Table	In_use	Name_locked
 DROP TABLE t1, t2;
+create table t1 (pk int primary key, data int) engine=innodb;
+insert t1 values (1,1),(2,2),(3,3);
+create table t2 (t1_pk int, foreign key (t1_pk) references t1 (pk)) engine=innodb;
+insert t2 values (1),(2);
+insert t2 values (10);
+ERROR 23000: Cannot add or update a child row: a foreign key constraint fails (`test`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`t1_pk`) REFERENCES `t1` (`pk`))
+flush tables;
+flush status;
+update t1 set data=10 where pk+1>10;
+show status like '%opened_tab%';
+Variable_name	Value
+Opened_table_definitions	1
+Opened_tables	1
+flush tables;
+flush status;
+update t2 set t1_pk=11 where t1_pk+1>10;
+show status like '%opened_tab%';
+Variable_name	Value
+Opened_table_definitions	1
+Opened_tables	1
+drop table t2, t1;

mysql-test/suite/innodb/t/foreign-keys.test

@@ -158,3 +158,26 @@ connection default;
 SET debug_sync='reset';
 SHOW OPEN TABLES FROM test;
 DROP TABLE t1, t2;
+
+#
+# MDEV-22180 Planner opens unnecessary tables when updated table is referenced by foreign keys
+#
+source include/have_innodb.inc;
+
+create table t1 (pk int primary key, data int) engine=innodb;
+insert t1 values (1,1),(2,2),(3,3);
+create table t2 (t1_pk int, foreign key (t1_pk) references t1 (pk)) engine=innodb;
+insert t2 values (1),(2);
+error ER_NO_REFERENCED_ROW_2;
+insert t2 values (10);
+flush tables;
+flush status;
+# with ON UPDATE RESTRICT child tables are not opened
+update t1 set data=10 where pk+1>10;
+show status like '%opened_tab%';
+flush tables;
+flush status;
+# neither are parent tables
+update t2 set t1_pk=11 where t1_pk+1>10;
+show status like '%opened_tab%';
+drop table t2, t1;

sql/table.h

@@ -1813,6 +1813,9 @@ struct TABLE_LIST
     open_type= routine ? OT_TEMPORARY_OR_BASE : OT_BASE_ONLY;
     belong_to_view= belong_to_view_arg;
     trg_event_map= trg_event_map_arg;
+    /* MDL is enough for read-only FK checks, we don't need the table */
+    if (prelocking_placeholder == FK && lock_type < TL_WRITE_ALLOW_WRITE)
+      open_strategy= OPEN_STUB;
 
     **last_ptr= this;
     prev_global= *last_ptr;