Fixed bug #24670: optimizations that are legal only for subqueries without tables

and no WHERE condition were applied for any subquery without tables.



mysql-test/r/subselect.result:
  Added a test case for bug #24670.
mysql-test/t/subselect.test:
  Added a test case for bug #24670.
sql/item_subselect.cc:
  Fixed bug #24670: optimizations that are legal only for subqueries without tables
  and no WHERE condition were applied for any subquery without tables.
  
  Removed an assertion that caused an abort for subqueries without tables and no 
  WHERE condition. 
  Blocked substitution of a single-row subquery without tables for the constant 
  row from its select list when the subquery contained a WHERE condition.
  This optimization is valid only for subquries without tables with no conditions.
  Any subquery without tables with WHERE clause returns NULL if the WHERE condition
  is FALSE. Erroneously it was always considered as non-nullable that could trigger 
  another optimization concerning IS NULL predicates which is applicable only for 
  non-nullable expressions and ultimately led to a wrong result returned by the outer
  query.
  Added a proper implementation of the virtual method may_be_null for class 
  subselect_single_select_engine.
sql/item_subselect.h:
  Fixed bug #24670: optimizations that are legal only for subqueries without tables
  and no WHERE condition were applied for any subquery without tables.
  Made method may_by_null for class subselect_engine vvirtual.

1 files changed, 13 insertions, 0 deletions

diff --git a/mysql-test/r/subselect.result b/mysql-test/r/subselect.result
index 89315342f5e..a4ea8e21d61 100644
--- a/mysql-test/r/subselect.result
+++ b/mysql-test/r/subselect.result
@@ -3013,3 +3013,16 @@ t3	CREATE TABLE `t3` (
   `a` datetime default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1,t2,t3;
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) > 0;
+a
+SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) IS NULL;
+a
+1
+2
+EXPLAIN SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+DROP TABLE t1;