Wrong result with LATERAL DERIVED

When populating the structure spl_opt_info for a TABLE, and evaluating a key_field for inclusion in spl_opt_info->added_key_fields, the null_rejecting attribute may be incorrectly set.

Our example from the test case

select STRAIGHT_JOIN count(*) FROM t1 tc

INNER JOIN

(

  SELECT a, b, c, max(d) maxd  FROM t1

  GROUP BY a, b, c

) c ON tc.a = c.a AND tc.b = c.b  AND tc.c <=> c.c

LEFT JOIN

(

  SELECT a, b, c, max(d) umaxd  FROM t1

 WHERE e = 'Y'

  GROUP BY a, b, c

) c2 ON tc.a = c2.a AND tc.b = c2.b  AND tc.c <=> c2.c

WHERE tc.d < c.maxd  AND NOT tc.d <=> c2.umaxd

Originally, this null_rejecting was assumed to be TRUE, then it was changed

      Item *real= key_field->val->real_item();

      if ((real->type() == Item::FIELD_ITEM) &&

            ((Item_field*)real)->field->maybe_null())

        added_key_field->null_rejecting= true;

      else

        added_key_field->null_rejecting= false;

which also incorrectly assumed that the added key field depended on whether the field was able to set to null.

In the above query, for the derived table c2, we do not want this attribute to be null rejecting for key usage related to our join conditions, and we do not want equality conditions pushed into this derived table during a lateral join.