Fix for bug lp:944706, task MDEV-193

The patch enables back constant subquery execution during
query optimization after it was disabled during the development
of MWL#89 (cost-based choice of IN-TO-EXISTS vs MATERIALIZATION).

The main idea is that constant subqueries are allowed to be executed
during optimization if their execution is not expensive.

The approach is as follows:
- Constant subqueries are recursively optimized in the beginning of
  JOIN::optimize of the outer query. This is done by the new method
  JOIN::optimize_constant_subqueries(). This is done so that the cost
  of executing these queries can be estimated.
- Optimization of the outer query proceeds normally. During this phase
  the optimizer may request execution of non-expensive constant subqueries.
  Each place where the optimizer may potentially execute an expensive
  expression is guarded with the predicate Item::is_expensive().
- The implementation of Item_subselect::is_expensive has been extended
  to use the number of examined rows (estimated by the optimizer) as a
  way to determine whether the subquery is expensive or not.
- The new system variable "expensive_subquery_limit" controls how many
  examined rows are considered to be not expensive. The default is 100.

In addition, multiple changes were needed to make this solution work
in the light of the changes made by MWL#89. These changes were needed
to fix various crashes and wrong results, and legacy bugs discovered
during development.

1 files changed, 6 insertions, 6 deletions

diff --git a/mysql-test/r/explain.result b/mysql-test/r/explain.result
index a6fa21ad800..863526c637e 100644
--- a/mysql-test/r/explain.result
+++ b/mysql-test/r/explain.result
@@ -260,7 +260,7 @@ FLUSH TABLES;
 EXPLAIN SELECT 1 FROM t1 WHERE a = (SELECT 1 FROM t1 t JOIN t2 WHERE b <= 1 AND t.a);
 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	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	no matching row in const table
 DROP TABLE t1, t2;
 #
 # Bug #48573: difference of index selection between rpm binary and
@@ -287,7 +287,7 @@ WHERE t1.f1 GROUP BY t1.f1));
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
 2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	
-2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	Using where
+2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	
 PREPARE stmt FROM
 'EXPLAIN SELECT 1 FROM t1
  WHERE 1 > ALL((SELECT t1.f1 FROM t1 RIGHT OUTER JOIN t1 a
@@ -297,12 +297,12 @@ EXECUTE stmt;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
 2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	
-2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	Using where
+2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	
 EXECUTE stmt;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
 2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	
-2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	Using where
+2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	
 DEALLOCATE PREPARE stmt;
 PREPARE stmt FROM
 'EXPLAIN SELECT 1 FROM t1
@@ -313,12 +313,12 @@ EXECUTE stmt;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
 2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	
-2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	Using where
+2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	
 EXECUTE stmt;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	
 2	SUBQUERY	a	system	NULL	NULL	NULL	NULL	1	
-2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	Using where
+2	SUBQUERY	t1	fulltext	f1_2,f1	f1	0		1	
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
 End of 5.1 tests.