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.

2 files changed, 40 insertions, 0 deletions

diff --git a/mysql-test/t/subselect.test b/mysql-test/t/subselect.test
index 6e98c064d94..d649c300ab5 100644
--- a/mysql-test/t/subselect.test
+++ b/mysql-test/t/subselect.test
@@ -304,6 +304,7 @@ SELECT (SELECT numeropost FROM t1 HAVING numreponse=a),numreponse FROM (SELECT *
 SELECT numreponse, (SELECT numeropost FROM t1 HAVING numreponse=a) FROM (SELECT * FROM t1) as a;
 SELECT numreponse, (SELECT numeropost FROM t1 HAVING numreponse=1) FROM (SELECT * FROM t1) as a;
 INSERT INTO t1 (numeropost,numreponse,pseudo) VALUES (1,1,'joce'),(1,2,'joce'),(1,3,'test');
+-- error ER_SUBQUERY_NO_1_ROW
 EXPLAIN EXTENDED SELECT numreponse FROM t1 WHERE numeropost='1' AND numreponse=(SELECT 1 FROM t1 WHERE numeropost='1');
 -- error ER_SUBQUERY_NO_1_ROW
 SELECT numreponse FROM t1 WHERE numeropost='1' AND numreponse=(SELECT 1 FROM t1 WHERE numeropost='1');
@@ -5529,5 +5530,36 @@ SELECT COUNT(f1), f2 IN (SELECT f1 FROM t1 WHERE f2 > 0) AS f4 FROM t2, t1 WHERE
 drop table t1,t2;
 
 
+--echo #
+--echo # LP BUG#944706 Query with impossible or constant subquery in WHERE or HAVING is not
+--echo # precomputed and thus not part of optimization
+--echo #
+
+CREATE TABLE t1 ( a VARCHAR(16), KEY (a) );
+INSERT INTO t1 VALUES ('Abilene'),('Akron'),('Albany'),('Albuquerque'),('Alexandria'),('Allentown'),
+('Amarillo'),('Anaheim'),('Anchorage'),('Ann Arbor'),('Arden-Arcade');
+
+EXPLAIN
+SELECT MAX( alias2.a ) AS field
+FROM t1 AS alias1, t1 AS alias2, t1 AS alias3
+WHERE alias1.a = alias2.a OR alias1.a = 'y'
+HAVING field>'B' AND ( 'Moscow' ) IN ( SELECT a FROM t1 );
+
+SELECT MAX( alias2.a ) AS field
+FROM t1 AS alias1, t1 AS alias2, t1 AS alias3
+WHERE alias1.a = alias2.a OR alias1.a = 'y'
+HAVING field>'B' AND ( 'Moscow' ) IN ( SELECT a FROM t1 );
+
+EXPLAIN
+SELECT MAX( alias2.a )
+FROM t1 AS alias1, t1 AS alias2, t1 AS alias3
+WHERE alias1.a = alias2.a OR ('Moscow') IN ( SELECT a FROM t1 );
+
+SELECT MAX( alias2.a )
+FROM t1 AS alias1, t1 AS alias2, t1 AS alias3
+WHERE alias1.a = alias2.a OR ('Moscow') IN ( SELECT a FROM t1 );
+
+drop table t1;
+
 --echo # return optimizer switch changed in the beginning of this test
 set optimizer_switch=@subselect_tmp;
diff --git a/mysql-test/t/subselect4.test b/mysql-test/t/subselect4.test
index 8915cdd6500..9ec282ba3db 100644
--- a/mysql-test/t/subselect4.test
+++ b/mysql-test/t/subselect4.test
@@ -977,6 +977,14 @@ SELECT * FROM t1 WHERE
         FROM t3 AS SQ1_t1 JOIN t3 AS SQ1_t3 ON SQ1_t3.f4
         GROUP BY SQ1_t1.f4));
 
+--error ER_SUBQUERY_NO_1_ROW
+SELECT * FROM t1 WHERE
+(SELECT f2 FROM t2
+ WHERE f4 <= ALL
+       (SELECT max(SQ1_t1.f4)
+        FROM t3 AS SQ1_t1 JOIN t3 AS SQ1_t3 ON SQ1_t3.f4
+        GROUP BY SQ1_t1.f4));
+
 drop table t1, t2, t3;
 
 --echo #