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

@@ -7240,6 +7240,22 @@ 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
 # End of 10.0 tests
 #

mysql-test/r/subselect_no_exists_to_in.result

@@ -7240,6 +7240,22 @@ 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
 # End of 10.0 tests
 #

mysql-test/r/subselect_no_mat.result

@@ -7233,6 +7233,22 @@ 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
 # End of 10.0 tests
 #

mysql-test/r/subselect_no_opts.result

@@ -7231,6 +7231,22 @@ 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
 # End of 10.0 tests
 #

mysql-test/r/subselect_no_scache.result

@@ -7246,6 +7246,22 @@ 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
 # End of 10.0 tests
 #

mysql-test/r/subselect_no_semijoin.result

@@ -7231,6 +7231,22 @@ 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
 # End of 10.0 tests
 #

mysql-test/t/subselect.test

@@ -6102,6 +6102,21 @@ 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
 --echo # End of 10.0 tests
 

sql/item.h

@@ -1614,6 +1614,11 @@ class Item: public Value_source,
   virtual bool limit_index_condition_pushdown_processor(void *arg) { return 0; }
   virtual bool exists2in_processor(void *arg) { return 0; }
   virtual bool find_selective_predicates_list_processor(void *arg) { return 0; }
+  bool cleanup_is_expensive_cache_processor(void *arg)
+  {
+    is_expensive_cache= (int8)(-1);
+    return 0;
+  }
 
   /* 
     TRUE if the expression depends only on the table indicated by tab_map

sql/sql_select.cc

@@ -1335,6 +1335,13 @@ JOIN::optimize_inner()
   if (optimize_constant_subqueries())
     DBUG_RETURN(1);
 
+  if (conds && conds->has_subquery())
+    (void) conds->walk(&Item::cleanup_is_expensive_cache_processor,
+                       0, (void *) 0);
+  if (having && having->has_subquery())
+    (void) having->walk(&Item::cleanup_is_expensive_cache_processor,
+			0, (void *) 0);
+
   if (setup_jtbm_semi_joins(this, join_list, &conds))
     DBUG_RETURN(1);