Patch for mdev-287: CHEAP SQ: A query with subquery in SELECT list, EXISTS,...

Patch for mdev-287: CHEAP SQ: A query with subquery in SELECT list, EXISTS, inner joins takes hundreds times longer

Analysis:

The fix for lp:944706 introduces early subquery optimization.
While a subquery is being optimized some of its predicates may be
removed. In the test case, the EXISTS subquery is constant, and is
evaluated to TRUE. As a result the whole OR is TRUE, and thus the
correlated condition "b = alias1.b" is optimized away. The subquery
becomes non-correlated.

The subquery cache is designed to work only for correlated subqueries.
If constant subquery optimization is disallowed, then the constant
subquery is not evaluated, the subquery remains correlated, and its
execution is cached. As a result execution is fast.

However, when the constant subquery was optimized away, it was neither
cached by the subquery cache, nor it was cached by the internal subquery
caching. The latter was due to the fact that the subquery still appeared
as correlated to the subselect_XYZ_engine::exec methods, and they
re-executed the subquery on each call to Item_subselect::exec.

Solution:

The solution is to update the correlated status of the subquery after it has
been optimized. This status consists of:
- st_select_lex::is_correlated
- Item_subselect::is_correlated
- SELECT_LEX::uncacheable
- SELECT_LEX_UNIT::uncacheable
The status is updated by st_select_lex::update_correlated_cache(), and its
caller st_select_lex::optimize_unflattened_subqueries. The solution relies
on the fact that the optimizer already called
st_select_lex::update_used_tables() for each subquery. This allows to
efficiently update the correlated status of each subquery without walking
the whole subquery tree.

Notice that his patch is an improvement over MySQL 5.6 and older, where
subqueries are not pre-optimized, and the above analysis is not possible.

mysql-test/r/derived_opt.result

@@ -161,24 +161,24 @@ prepare stmt1 from @stmt ;
 execute stmt1 ;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+5	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 execute stmt1 ;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+5	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 explain SELECT (SELECT SUM(c1 + c12 + 0.0) FROM t2 where (t1.c2 - 0e-3) = t2.c2 GROUP BY t1.c15 LIMIT 1) as scalar_s, exists (select 1.0e+0 from t2 where t2.c3 * 9.0000000000 = t1.c4) as exists_s, c5 * 4 in (select c6 + 0.3e+1 from t2) as in_s, (c7 - 4, c8 - 4) in (select c9 + 4.0, c10 + 40e-1 from t2) as in_row_s FROM t1, (select c25 x, c32 y from t2) tt WHERE x * 1 = c25;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+5	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 deallocate prepare stmt1;
 drop tables t1,t2;
 set @@optimizer_switch=@save_optimizer_switch;

mysql-test/r/explain.result

@@ -225,10 +225,10 @@ INSERT INTO t2 VALUES (NULL), (0);
 EXPLAIN EXTENDED SELECT (SELECT 1 FROM t2 WHERE d = c) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	100.00	
-2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 Warnings:
 Note	1276	Field or reference 'test.t1.c' of SELECT #2 was resolved in SELECT #1
-Note	1003	select <expr_cache><NULL>((select 1 from `test`.`t2` where (`test`.`t2`.`d` = NULL))) AS `(SELECT 1 FROM t2 WHERE d = c)` from dual
+Note	1003	select (select 1 from `test`.`t2` where (`test`.`t2`.`d` = NULL)) AS `(SELECT 1 FROM t2 WHERE d = c)` from dual
 DROP TABLE t1, t2;
 #
 # Bug#30302: Tables that were optimized away are printed in the

mysql-test/r/index_merge_innodb.result

@@ -654,7 +654,7 @@ EXPLAIN SELECT t1.f1 FROM t1
 WHERE (SELECT COUNT(*) FROM t2 WHERE t2.f3 = 'h' AND t2.f2 = t1.f1) = 0 AND t1.f1 = 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	const	PRIMARY	PRIMARY	4	const	1	Using index
-2	DEPENDENT SUBQUERY	t2	index_merge	f2,f3	f3,f2	2,5	NULL	1	Using intersect(f3,f2); Using where; Using index
+2	SUBQUERY	t2	index_merge	f2,f3	f3,f2	2,5	NULL	1	Using intersect(f3,f2); Using where; Using index
 DROP TABLE t1,t2;
 # 
 # BUG#56862/640419: Wrong result with sort_union index merge when one

mysql-test/r/index_merge_myisam.result

@@ -1487,7 +1487,7 @@ EXPLAIN SELECT t1.f1 FROM t1
 WHERE (SELECT COUNT(*) FROM t2 WHERE t2.f3 = 'h' AND t2.f2 = t1.f1) = 0 AND t1.f1 = 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	PRIMARY	NULL	NULL	NULL	1	
-2	DEPENDENT SUBQUERY	t2	ref	f2,f3	f2	5	const	1	Using where
+2	SUBQUERY	t2	ref	f2,f3	f2	5	const	1	Using where
 DROP TABLE t1,t2;
 create table t0 (a int);
 insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

mysql-test/r/join_outer.result

@@ -1872,7 +1872,7 @@ SELECT t2.a FROM t1 LEFT JOIN t2 ON (6) IN (SELECT a FROM t3);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	100.00	
-2	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+2	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 Warnings:
 Note	1003	select NULL AS `a` from `test`.`t2` where 1
 DROP TABLE t1,t2,t3;

mysql-test/r/join_outer_jcl6.result

@@ -1883,7 +1883,7 @@ SELECT t2.a FROM t1 LEFT JOIN t2 ON (6) IN (SELECT a FROM t3);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	100.00	
 1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	100.00	
-2	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+2	SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 Warnings:
 Note	1003	select NULL AS `a` from `test`.`t2` where 1
 DROP TABLE t1,t2,t3;

mysql-test/r/ps.result

@@ -160,26 +160,26 @@ execute stmt1 ;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 6	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+5	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 execute stmt1 ;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 6	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+5	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 explain SELECT (SELECT SUM(c1 + c12 + 0.0) FROM t2 where (t1.c2 - 0e-3) = t2.c2 GROUP BY t1.c15 LIMIT 1) as scalar_s, exists (select 1.0e+0 from t2 where t2.c3 * 9.0000000000 = t1.c4) as exists_s, c5 * 4 in (select c6 + 0.3e+1 from t2) as in_s, (c7 - 4, c8 - 4) in (select c9 + 4.0, c10 + 40e-1 from t2) as in_row_s FROM t1, (select c25 x, c32 y from t2) tt WHERE x * 1 = c25;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 6	DERIVED	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
-5	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-4	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-3	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+5	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+4	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+3	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 deallocate prepare stmt1;
 set optimizer_switch=@tmp_optimizer_switch;
 drop tables t1,t2;

mysql-test/r/subselect_extra_no_semijoin.result

@@ -46,7 +46,7 @@ select * from t1
 where id in (select id from t1 as x1 where (t1.cur_date is null));
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	100.00	
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 Warnings:
 Note	1276	Field or reference 'test.t1.cur_date' of SELECT #2 was resolved in SELECT #1
 Note	1003	select 1 AS `id`,'2007-04-25 18:30:22' AS `cur_date` from dual where <expr_cache><1>(<in_optimizer>(1,<exists>(select `test`.`x1`.`id` from `test`.`t1` `x1` where 0)))
@@ -58,7 +58,7 @@ select * from t2
 where id in (select id from t2 as x1 where (t2.cur_date is null));
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	100.00	
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 Warnings:
 Note	1276	Field or reference 'test.t2.cur_date' of SELECT #2 was resolved in SELECT #1
 Note	1003	select 1 AS `id`,'2007-04-25' AS `cur_date` from dual where <expr_cache><1>(<in_optimizer>(1,<exists>(select `test`.`x1`.`id` from `test`.`t2` `x1` where 0)))

mysql-test/r/subselect_mat.result

@@ -1163,7 +1163,7 @@ set @@optimizer_switch='materialization=off,in_to_exists=on';
 explain select min(a1) from t1 where 7 in (select max(b1) from t2 group by b1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 select min(a1) from t1 where 7 in (select max(b1) from t2 group by b1);
 min(a1)
 NULL
@@ -1182,7 +1182,7 @@ set @@optimizer_switch='materialization=off,in_to_exists=on';
 explain select min(a1) from t1 where 7 in (select b1 from t2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 select min(a1) from t1 where 7 in (select b1 from t2);
 min(a1)
 NULL
@@ -1190,7 +1190,7 @@ NULL
 explain select min(a1) from t1 where 7 in (select b1 from t2) or 2> 4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 select min(a1) from t1 where 7 in (select b1 from t2) or 2> 4;
 min(a1)
 NULL
@@ -2162,7 +2162,7 @@ EXPLAIN
 SELECT (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
-3	DEPENDENT SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
 2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 SELECT (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1);
 (SELECT f3a FROM t3) NOT IN (SELECT f1a FROM t1)
@@ -2179,7 +2179,7 @@ EXPLAIN
 SELECT (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
-3	DEPENDENT SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
 2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 SELECT (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1);
 (SELECT f3a, f3b FROM t3) NOT IN (SELECT f1a, f1b FROM t1)
@@ -2234,7 +2234,7 @@ EXPLAIN EXTENDED
 SELECT MAX(t1.b) AS max_res FROM t1 WHERE (9) IN (SELECT a FROM t2);
 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	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 Warnings:
 Note	1003	select max(`test`.`t1`.`b`) AS `max_res` from `test`.`t1` where 0
 DROP TABLE t1,t2;

mysql-test/r/subselect_mat_cost.result

@@ -496,7 +496,7 @@ from City
 where City.population > 10000000;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	City	range	Population	Population	5	NULL	4	Using index condition; Rowid-ordered scan
-2	DEPENDENT SUBQUERY	Country	index_subquery	CountryCapital	CountryCapital	5	func	2	Using index; Using where
+2	SUBQUERY	Country	index_subquery	CountryCapital	CountryCapital	5	func	2	Using index; Using where
 select Name, City.id in (select capital from Country where capital is not null) as is_capital
 from City
 where City.population > 10000000;

mysql-test/r/subselect_mat_cost_bugs.result

@@ -124,7 +124,7 @@ FROM t3 RIGHT JOIN t1 ON t1.pk = t3.f1
 WHERE t3.f3 OR ( 3 ) IN ( SELECT f2 FROM t2 );
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 SELECT t1.*
 FROM t3 RIGHT JOIN t1 ON t1.pk = t3.f1
 WHERE t3.f3 OR ( 3 ) IN ( SELECT f2 FROM t2 );

mysql-test/r/subselect_partial_match.result

@@ -879,7 +879,7 @@ EXPLAIN
 SELECT * FROM t1 WHERE (6, 4 ) NOT IN (SELECT b, a FROM t2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
-2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
 SELECT * FROM t1 WHERE (6, 4 ) NOT IN (SELECT b, a FROM t2);
 c
 0
@@ -888,7 +888,7 @@ EXPLAIN
 SELECT * FROM t1 WHERE (6, 4 ) NOT IN (SELECT a, b FROM t2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
-2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
 SELECT * FROM t1 WHERE (6, 4 ) NOT IN (SELECT a, b FROM t2);
 c
 0
@@ -913,7 +913,7 @@ set @@optimizer_switch='in_to_exists=on,materialization=off';
 EXPLAIN SELECT * FROM t2 WHERE ( 3 , 1 ) NOT IN ( SELECT f1 , f2 FROM t1 );
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	
-2	DEPENDENT SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
 SELECT * FROM t2 WHERE ( 3 , 1 ) NOT IN ( SELECT f1 , f2 FROM t1 );
 f3
 5

mysql-test/r/subselect_sj_mat.result

@@ -1198,7 +1198,7 @@ set @@optimizer_switch='materialization=off,in_to_exists=on';
 explain select min(a1) from t1 where 7 in (select max(b1) from t2 group by b1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 select min(a1) from t1 where 7 in (select max(b1) from t2 group by b1);
 min(a1)
 NULL
@@ -1207,7 +1207,7 @@ set @@optimizer_switch='semijoin=off';
 explain select min(a1) from t1 where 7 in (select b1 from t2);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 select min(a1) from t1 where 7 in (select b1 from t2);
 min(a1)
 NULL
@@ -1224,7 +1224,7 @@ NULL
 explain select min(a1) from t1 where 7 in (select b1 from t2) or 2> 4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 select min(a1) from t1 where 7 in (select b1 from t2) or 2> 4;
 min(a1)
 NULL

mysql-test/t/subselect.test

@@ -5580,5 +5580,49 @@ WHERE a1 = (SELECT COUNT(*) FROM t1 WHERE a1 IN (SELECT a1 FROM t1, t2));
 
 drop table t1, t2;
 
+--echo #
+--echo # MDEV-287 CHEAP SQ: A query with subquery in SELECT list, EXISTS,
+--echo # inner joins takes hundreds times longer
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(7);
+
+CREATE TABLE t2 (b INT);
+INSERT INTO t2 VALUES (4),(5);
+
+CREATE TABLE t3 (c INT);
+INSERT INTO t3 VALUES (8),(3);
+
+set @@expensive_subquery_limit= 0;
+
+EXPLAIN
+SELECT (SELECT MIN(b) FROM t1, t2 WHERE b = a AND (b = alias1.b OR EXISTS (SELECT * FROM t3)))
+FROM t2 alias1, t1 alias2, t1 alias3;
+
+flush status;
+
+SELECT (SELECT MIN(b) FROM t1, t2 WHERE b = a AND (b = alias1.b OR EXISTS (SELECT * FROM t3)))
+FROM t2 alias1, t1 alias2, t1 alias3;
+
+show status like "subquery_cache%";
+show status like '%Handler_read%';
+
+set @@expensive_subquery_limit= default;
+
+EXPLAIN
+SELECT (SELECT MIN(b) FROM t1, t2 WHERE b = a AND (b = alias1.b OR EXISTS (SELECT * FROM t3)))
+FROM t2 alias1, t1 alias2, t1 alias3;
+
+flush status;
+
+SELECT (SELECT MIN(b) FROM t1, t2 WHERE b = a AND (b = alias1.b OR EXISTS (SELECT * FROM t3)))
+FROM t2 alias1, t1 alias2, t1 alias3;
+
+show status like "subquery_cache%";
+show status like '%Handler_read%';
+
+drop table t1, t2, t3;
+
 --echo # return optimizer switch changed in the beginning of this test
 set optimizer_switch=@subselect_tmp;

sql/sql_lex.cc

@@ -3443,6 +3443,7 @@ bool st_select_lex::optimize_unflattened_subqueries(bool const_only)
       }
 
       bool empty_union_result= true;
+      bool is_correlated_unit= false;
       /*
         If the subquery is a UNION, optimize all the subqueries in the UNION. If
         there is no UNION, then the loop will execute once for the subquery.
@@ -3467,6 +3468,8 @@ bool st_select_lex::optimize_unflattened_subqueries(bool const_only)
           inner_join->select_options|= SELECT_DESCRIBE;
         }
         res= inner_join->optimize();
+        sl->update_correlated_cache();
+        is_correlated_unit|= sl->is_correlated;
         inner_join->select_options= save_options;
         un->thd->lex->current_select= save_select;
         if (empty_union_result)
@@ -3482,6 +3485,9 @@ bool st_select_lex::optimize_unflattened_subqueries(bool const_only)
       }
       if (empty_union_result)
         subquery_predicate->no_rows_in_result();
+      if (!is_correlated_unit)
+        un->uncacheable&= ~UNCACHEABLE_DEPENDENT;
+      subquery_predicate->is_correlated= is_correlated_unit;
     }
   }
   return FALSE;
@@ -3850,6 +3856,61 @@ void SELECT_LEX::update_used_tables()
 }
 
 
+/**
+  @brief
+  Update is_correlated cache for this select
+
+  @details
+*/
+
+void st_select_lex::update_correlated_cache()
+{
+  TABLE_LIST *tl;
+  List_iterator<TABLE_LIST> ti(leaf_tables);
+
+  is_correlated= false;
+
+  while ((tl= ti++))
+  {
+    if (tl->on_expr)
+      is_correlated|= test(tl->on_expr->used_tables() & OUTER_REF_TABLE_BIT);
+    for (TABLE_LIST *embedding= tl->embedding ; embedding ;
+         embedding= embedding->embedding)
+    {
+      if (embedding->on_expr)
+        is_correlated|= test(embedding->on_expr->used_tables() &
+                            OUTER_REF_TABLE_BIT);
+    }
+  }
+
+  if (join->conds)
+    is_correlated|= test(join->conds->used_tables() & OUTER_REF_TABLE_BIT);
+
+  if (join->having)
+    is_correlated|= test(join->having->used_tables() & OUTER_REF_TABLE_BIT);
+
+  if (join->tmp_having)
+    is_correlated|= test(join->tmp_having->used_tables() & OUTER_REF_TABLE_BIT);
+
+  Item *item;
+  List_iterator_fast<Item> it(join->fields_list);
+  while ((item= it++))
+    is_correlated|= test(item->used_tables() & OUTER_REF_TABLE_BIT);
+
+  for (ORDER *order= group_list.first; order; order= order->next)
+    is_correlated|= test((*order->item)->used_tables() & OUTER_REF_TABLE_BIT);
+
+  if (!master_unit()->is_union())
+  {
+    for (ORDER *order= order_list.first; order; order= order->next)
+      is_correlated|= test((*order->item)->used_tables() & OUTER_REF_TABLE_BIT);
+  }
+
+  if (!is_correlated)
+    uncacheable&= ~UNCACHEABLE_DEPENDENT;
+}
+
+
 /**
   Set the EXPLAIN type for this subquery.
 */

sql/sql_lex.h

@@ -1018,6 +1018,7 @@ public:
   void mark_as_belong_to_derived(TABLE_LIST *derived);
   void increase_derived_records(ha_rows records);
   void update_used_tables();
+  void update_correlated_cache();
   void mark_const_derived(bool empty);
 
   bool save_leaf_tables(THD *thd);