BUG#16798: Inapplicable ref_or_null query plan and bad query result on random occasions

The bug was as follows: When merge_key_fields() encounters "t.key=X OR t.key=Y" it will 
try to join them into ref_or_null access via "t.key=X OR NULL". In order to make this 
inference it checks if Y<=>NULL, ignoring the fact that value of Y may be not yet known.

The fix is that the check if Y<=>NULL is made only if value of Y is known (i.e. it is a
constant).
TODO: When merging to 5.0, replace used_tables() with const_item() everywhere in merge_key_fields().

mysql-test/r/innodb_mysql.result

-drop table if exists t1;
+drop table if exists t1,t2;
+create table t1 (
+c_id int(11) not null default '0',
+org_id int(11) default null,
+unique key contacts$c_id (c_id),
+key contacts$org_id (org_id)
+) engine=innodb;
+insert into t1 values 
+(2,null),(120,null),(141,null),(218,7), (128,1),
+(151,2),(234,2),(236,2),(243,2),(255,2),(259,2),(232,3),(235,3),(238,3),
+(246,3),(253,3),(269,3),(285,3),(291,3),(293,3),(131,4),(230,4),(231,4);
+create table t2 (
+slai_id int(11) not null default '0',
+owner_tbl int(11) default null,
+owner_id int(11) default null,
+sla_id int(11) default null,
+inc_web int(11) default null,
+inc_email int(11) default null,
+inc_chat int(11) default null,
+inc_csr int(11) default null,
+inc_total int(11) default null,
+time_billed int(11) default null,
+activedate timestamp null default null,
+expiredate timestamp null default null,
+state int(11) default null,
+sla_set int(11) default null,
+unique key t2$slai_id (slai_id),
+key t2$owner_id (owner_id),
+key t2$sla_id (sla_id)
+) engine=innodb;
+insert into t2(slai_id, owner_tbl, owner_id, sla_id) values
+(1,3,1,1), (3,3,10,2), (4,3,3,6), (5,3,2,5), (6,3,8,3), (7,3,9,7),
+(8,3,6,8), (9,3,4,9), (10,3,5,10), (11,3,11,11), (12,3,7,12);
+flush tables;
+select si.slai_id
+from t1 c join t2 si on
+((si.owner_tbl = 3 and si.owner_id = c.org_id) or 
+( si.owner_tbl = 2 and si.owner_id = c.c_id)) 
+where 
+c.c_id = 218 and expiredate is null;
+slai_id
+12
+select * from t1 where org_id is null;
+c_id	org_id
+2	NULL
+120	NULL
+141	NULL
+select si.slai_id
+from t1 c join t2 si on
+((si.owner_tbl = 3 and si.owner_id = c.org_id) or 
+( si.owner_tbl = 2 and si.owner_id = c.c_id)) 
+where 
+c.c_id = 218 and expiredate is null;
+slai_id
+12
+drop table t1, t2;

mysql-test/t/innodb_mysql.test

 -- source include/have_innodb.inc
 
 --disable_warnings
-drop table if exists t1;
+drop table if exists t1,t2;
 --enable_warnings
+
+# BUG#16798: Uninitialized row buffer reads in ref-or-null optimizer
+# (repeatable only w/innodb).
+create table t1 (
+  c_id int(11) not null default '0',
+  org_id int(11) default null,
+  unique key contacts$c_id (c_id),
+  key contacts$org_id (org_id)
+) engine=innodb;
+insert into t1 values 
+  (2,null),(120,null),(141,null),(218,7), (128,1),
+  (151,2),(234,2),(236,2),(243,2),(255,2),(259,2),(232,3),(235,3),(238,3),
+  (246,3),(253,3),(269,3),(285,3),(291,3),(293,3),(131,4),(230,4),(231,4);
+
+create table t2 (
+  slai_id int(11) not null default '0',
+  owner_tbl int(11) default null,
+  owner_id int(11) default null,
+  sla_id int(11) default null,
+  inc_web int(11) default null,
+  inc_email int(11) default null,
+  inc_chat int(11) default null,
+  inc_csr int(11) default null,
+  inc_total int(11) default null,
+  time_billed int(11) default null,
+  activedate timestamp null default null,
+  expiredate timestamp null default null,
+  state int(11) default null,
+  sla_set int(11) default null,
+  unique key t2$slai_id (slai_id),
+  key t2$owner_id (owner_id),
+  key t2$sla_id (sla_id)
+) engine=innodb;
+insert into t2(slai_id, owner_tbl, owner_id, sla_id) values
+  (1,3,1,1), (3,3,10,2), (4,3,3,6), (5,3,2,5), (6,3,8,3), (7,3,9,7),
+  (8,3,6,8), (9,3,4,9), (10,3,5,10), (11,3,11,11), (12,3,7,12);
+
+flush tables;
+select si.slai_id
+from t1 c join t2 si on
+  ((si.owner_tbl = 3 and si.owner_id = c.org_id) or 
+   ( si.owner_tbl = 2 and si.owner_id = c.c_id)) 
+where 
+  c.c_id = 218 and expiredate is null;
+  
+select * from t1 where org_id is null;
+select si.slai_id
+from t1 c join t2 si on
+  ((si.owner_tbl = 3 and si.owner_id = c.org_id) or 
+   ( si.owner_tbl = 2 and si.owner_id = c.c_id)) 
+where 
+  c.c_id = 218 and expiredate is null;
+
+drop table t1, t2;

sql/sql_select.cc

@@ -2090,7 +2090,8 @@ merge_key_fields(KEY_FIELD *start,KEY_FIELD *new_fields,KEY_FIELD *end,
                                 new_fields->null_rejecting);
 	}
 	else if (old->eq_func && new_fields->eq_func &&
-		 (old->val->is_null() || new_fields->val->is_null()))
+		 ((!old->val->used_tables() && old->val->is_null()) || 
+                  new_fields->val->is_null()))
 	{
 	  /* field = expression OR field IS NULL */
 	  old->level= and_level;