MDEV-16820 Lost 'Impossible where' from query with inexpensive subquery

This patch fixes another problem introduced by the patch for mdev-4817.
The latter changed Item_cond::fix_fields() in such a way that it could
call the virtual method is_expensive(). With the first its call
the method saves the result in Item::is_expensive_cache. For all next
calls the method returns the result from this cache. So if the item
once was determined as expensive the method always returns true.
For subqueries it's not good, because non-optimized subqueries always
is considered as expensive.
It means that the cache should be invalidated after the call of
optimize_constant_subqueries().

mysql-test/r/subselect.result

@@ -7171,4 +7171,20 @@ a
 5
 SET @@optimizer_switch= @optimiser_switch_save;
 DROP TABLE t1, t2, t3;
+#
+# MDEV-16820: impossible where with inexpensive subquery
+#
+create table t1 (a int) engine=myisam;
+insert into t1 values (3), (1), (7);
+create table t2 (b int, index idx(b));
+insert into t2 values (2), (5), (3), (2);
+explain select * from t1 where (select max(b) from t2) = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+explain select * from t1 where (select max(b) from t2) = 10 and t1.a > 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+drop table t1,t2;
 End of 5.5 tests

mysql-test/r/subselect_no_mat.result

@@ -7168,6 +7168,22 @@ a
 5
 SET @@optimizer_switch= @optimiser_switch_save;
 DROP TABLE t1, t2, t3;
+#
+# MDEV-16820: impossible where with inexpensive subquery
+#
+create table t1 (a int) engine=myisam;
+insert into t1 values (3), (1), (7);
+create table t2 (b int, index idx(b));
+insert into t2 values (2), (5), (3), (2);
+explain select * from t1 where (select max(b) from t2) = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+explain select * from t1 where (select max(b) from t2) = 10 and t1.a > 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+drop table t1,t2;
 End of 5.5 tests
 set optimizer_switch=default;
 select @@optimizer_switch like '%materialization=on%';

mysql-test/r/subselect_no_opts.result

@@ -7166,5 +7166,21 @@ a
 5
 SET @@optimizer_switch= @optimiser_switch_save;
 DROP TABLE t1, t2, t3;
+#
+# MDEV-16820: impossible where with inexpensive subquery
+#
+create table t1 (a int) engine=myisam;
+insert into t1 values (3), (1), (7);
+create table t2 (b int, index idx(b));
+insert into t2 values (2), (5), (3), (2);
+explain select * from t1 where (select max(b) from t2) = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+explain select * from t1 where (select max(b) from t2) = 10 and t1.a > 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+drop table t1,t2;
 End of 5.5 tests
 set @optimizer_switch_for_subselect_test=null;

mysql-test/r/subselect_no_scache.result

@@ -7177,6 +7177,22 @@ a
 5
 SET @@optimizer_switch= @optimiser_switch_save;
 DROP TABLE t1, t2, t3;
+#
+# MDEV-16820: impossible where with inexpensive subquery
+#
+create table t1 (a int) engine=myisam;
+insert into t1 values (3), (1), (7);
+create table t2 (b int, index idx(b));
+insert into t2 values (2), (5), (3), (2);
+explain select * from t1 where (select max(b) from t2) = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+explain select * from t1 where (select max(b) from t2) = 10 and t1.a > 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+drop table t1,t2;
 End of 5.5 tests
 set optimizer_switch=default;
 select @@optimizer_switch like '%subquery_cache=on%';

mysql-test/r/subselect_no_semijoin.result

@@ -7166,6 +7166,22 @@ a
 5
 SET @@optimizer_switch= @optimiser_switch_save;
 DROP TABLE t1, t2, t3;
+#
+# MDEV-16820: impossible where with inexpensive subquery
+#
+create table t1 (a int) engine=myisam;
+insert into t1 values (3), (1), (7);
+create table t2 (b int, index idx(b));
+insert into t2 values (2), (5), (3), (2);
+explain select * from t1 where (select max(b) from t2) = 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+explain select * from t1 where (select max(b) from t2) = 10 and t1.a > 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+drop table t1,t2;
 End of 5.5 tests
 set @optimizer_switch_for_subselect_test=null;
 set @join_cache_level_for_subselect_test=NULL;

mysql-test/t/subselect.test

@@ -6060,4 +6060,19 @@ and t1.a in (select `test`.`t3`.`c` from `test`.`t3`);
 SET @@optimizer_switch= @optimiser_switch_save;
 DROP TABLE t1, t2, t3;
 
+--echo #
+--echo # MDEV-16820: impossible where with inexpensive subquery
+--echo #
+
+create table t1 (a int) engine=myisam;
+insert into t1 values (3), (1), (7);
+
+create table t2 (b int, index idx(b));
+insert into t2 values (2), (5), (3), (2);
+
+explain select * from t1 where (select max(b) from t2) = 10;
+explain select * from t1 where (select max(b) from t2) = 10 and t1.a > 3;
+
+drop table t1,t2;
+
 --echo End of 5.5 tests

sql/item.h

@@ -1234,6 +1234,11 @@ class Item {
   { 
     return FALSE;
   }
+  bool cleanup_is_expensive_cache_processor(uchar *arg)
+  {
+    is_expensive_cache= (int8)(-1);
+    return 0;
+  }
 
   /* To call bool function for all arguments */
   struct bool_func_call_args

sql/sql_select.cc

@@ -1098,6 +1098,13 @@ JOIN::optimize()
   if (optimize_constant_subqueries())
     DBUG_RETURN(1);
 
+  if (conds && conds->has_subquery())
+    (void) conds->walk(&Item::cleanup_is_expensive_cache_processor,
+                       0, (uchar*)0);
+  if (having && having->has_subquery())
+    (void) having->walk(&Item::cleanup_is_expensive_cache_processor,
+                       0, (uchar*)0);
+
   if (setup_jtbm_semi_joins(this, join_list, &conds))
     DBUG_RETURN(1);