bugfix: join a=b where cast(a as type_of_b) can produce NULL

optimizer implicitly assumed that if `a` in `a=b` is not NULL,
then it's safe to convert `a` to the type of `b` and search the
result in the index(b).

which is not always the case, as converting a non-null value
to a different type might produce NULL. And searching for NULL
in the index might find NULL there, so NULL will be equal to NULL,
making `a=b` behave as if it was `a<=>b`

mysql-test/main/subselect_cache.result

@@ -1698,7 +1698,7 @@ Subquery_cache_miss	18
 show status like '%Handler_read%';
 Variable_name	Value
 Handler_read_first	0
-Handler_read_key	32
+Handler_read_key	29
 Handler_read_last	0
 Handler_read_next	0
 Handler_read_prev	0
@@ -1763,7 +1763,7 @@ Subquery_cache_miss	10
 show status like '%Handler_read%';
 Variable_name	Value
 Handler_read_first	0
-Handler_read_key	13
+Handler_read_key	12
 Handler_read_last	0
 Handler_read_next	0
 Handler_read_prev	0

plugin/type_uuid/mysql-test/type_uuid/order.test

@@ -16,7 +16,21 @@ select * from t2;
 select * from t2 order by a;
 show create table t2;
 
-select * from t1, t2 where t1.a=t2.a;
+explain select * from t1 left join t2 on (t1.a=t2.a);
+--sorted_result
+select * from t1 left join t2 on (t1.a=t2.a);
+
+explain select * from t1 left join t2 on (t1.a<=>t2.a);
+--sorted_result
+select * from t1 left join t2 on (t1.a<=>t2.a);
+
+explain select * from t2 left join t1 on (t1.a=t2.a);
+--sorted_result
+select * from t2 left join t1 on (t1.a=t2.a);
+
+explain select * from t2 left join t1 on (t1.a<=>t2.a);
+--sorted_result
+select * from t2 left join t1 on (t1.a<=>t2.a);
 
 --sorted_result
 select * from t1 union select * from t2;

sql/sql_select.cc

@@ -25562,7 +25562,8 @@ cp_buffer_from_ref(THD *thd, TABLE *table, TABLE_REF *ref)
   thd->count_cuted_fields= CHECK_FIELD_IGNORE;
   for (store_key **copy=ref->key_copy ; *copy ; copy++)
   {
-    if ((*copy)->copy(thd) & 1)
+    if ((*copy)->copy(thd) & 1 ||
+        (ref->null_rejecting && (*copy)->null_key))
     {
       result= 1;
       break;