Fixed bug mdev-11859.

As the function Item_subselect::fix_fields does it the function
Item_subselect::update_used_tables must ignore UNCACHEABLE_EXPLAIN
when deciding whether the subquery item should be considered as a
constant item.

mysql-test/r/ps.result

@@ -4103,4 +4103,74 @@ NULL
 NULL
 deallocate prepare stmt;
 drop table t1,t2,t3,t4;
+#
+# MDEV-11859: the plans for the first and the second executions
+#             of PS are not the same
+#
+create table t1 (id int, c varchar(3), key idx(c))engine=myisam;
+insert into t1 values (3,'bar'), (1,'xxx'), (2,'foo'), (5,'yyy');
+prepare stmt1 from
+"explain extended
+ select * from t1 where (1, 2) in ( select 3, 4 ) or c = 'foo'";
+execute stmt1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t1	ref	idx	idx	6	const	1	100.00	Using index condition
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1003	select `test`.`t1`.`id` AS `id`,`test`.`t1`.`c` AS `c` from `test`.`t1` where (`test`.`t1`.`c` = 'foo')
+execute stmt1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t1	ref	idx	idx	6	const	1	100.00	Using index condition
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1003	select `test`.`t1`.`id` AS `id`,`test`.`t1`.`c` AS `c` from `test`.`t1` where (`test`.`t1`.`c` = 'foo')
+deallocate prepare stmt1;
+prepare stmt1 from
+"select * from t1 where (1, 2) in ( select 3, 4 ) or c = 'foo'";
+flush status;
+execute stmt1;
+id	c
+2	foo
+show status like '%Handler_read%';
+Variable_name	Value
+Handler_read_first	0
+Handler_read_key	1
+Handler_read_last	0
+Handler_read_next	1
+Handler_read_prev	0
+Handler_read_rnd	0
+Handler_read_rnd_deleted	0
+Handler_read_rnd_next	0
+flush status;
+execute stmt1;
+id	c
+2	foo
+show status like '%Handler_read%';
+Variable_name	Value
+Handler_read_first	0
+Handler_read_key	1
+Handler_read_last	0
+Handler_read_next	1
+Handler_read_prev	0
+Handler_read_rnd	0
+Handler_read_rnd_deleted	0
+Handler_read_rnd_next	0
+deallocate prepare stmt1;
+prepare stmt2 from
+"explain extended
+ select * from t1 where (1, 2) in ( select 3, 4 )";
+execute stmt2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1003	select `test`.`t1`.`id` AS `id`,`test`.`t1`.`c` AS `c` from `test`.`t1` where 0
+execute stmt2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+Warnings:
+Note	1003	select `test`.`t1`.`id` AS `id`,`test`.`t1`.`c` AS `c` from `test`.`t1` where 0
+deallocate prepare stmt2;
+drop table t1;
 # End of 5.5 tests

mysql-test/t/ps.test

@@ -3680,5 +3680,38 @@ EXECUTE stmt;
 deallocate prepare stmt;
 drop table t1,t2,t3,t4;
 
+--echo #
+--echo # MDEV-11859: the plans for the first and the second executions
+--echo #             of PS are not the same
+--echo #
+
+create table t1 (id int, c varchar(3), key idx(c))engine=myisam;
+insert into t1 values (3,'bar'), (1,'xxx'), (2,'foo'), (5,'yyy');
+
+prepare stmt1 from
+"explain extended
+ select * from t1 where (1, 2) in ( select 3, 4 ) or c = 'foo'";
+execute stmt1;
+execute stmt1;
+deallocate prepare stmt1;
+
+prepare stmt1 from
+"select * from t1 where (1, 2) in ( select 3, 4 ) or c = 'foo'";
+flush status;
+execute stmt1;
+show status like '%Handler_read%';
+flush status;
+execute stmt1;
+show status like '%Handler_read%';
+deallocate prepare stmt1;
+
+prepare stmt2 from
+"explain extended
+ select * from t1 where (1, 2) in ( select 3, 4 )";
+execute stmt2;
+execute stmt2;
+deallocate prepare stmt2;
+
+drop table t1;
 
 --echo # End of 5.5 tests

sql/item_subselect.cc

@@ -878,7 +878,7 @@ void Item_subselect::update_used_tables()
   if (!forced_const)
   {
     recalc_used_tables(parent_select, FALSE);
-    if (!engine->uncacheable())
+    if (!(engine->uncacheable() & ~UNCACHEABLE_EXPLAIN))
     {
       // did all used tables become static?
       if (!(used_tables_cache & ~engine->upper_select_const_tables()))