If join condition is of the form <t2.key>=<t1.no_key> then the server
performs no index look-ups when looking for matching rows of t2 for
the rows from t1 with t1.no_key=NULL. It happens because the function
add_not_null_conds() injects an additional condition of the form 
IS NOT NULL(<t1.no_key>) into the WHERE condition.
However if the join condition was of the form <t.key>=<outer_ref> no
additional null rejecting predicate was generated. This could lead
to extra records in the result set if the value of <outer_ref> happened
to be NULL.
The new code injects null rejecting predicates of the form 
IS NOT NULL(<outer_ref>) and evaluates them before the first row
the subquery is constructed.