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[Sergey Petrunya <psergey@xxxxxxxxxxxx>]

Hello Igor,

I've discovered the following: 

Consider a testcase for https://bugs.launchpad.net/maria/+bug/803365.

There we have a query that's using a derived table that's on the inner side 
of an outer join:

SELECT *
FROM t1
WHERE t1.f1 IN (
        SELECT t2.f2
        FROM t2
        LEFT JOIN (
                SELECT *
                FROM t3
        ) AS alias1
        ON alias1.f3 = t2.f2
);

If I follow it in debugger to right after simplify_joins(), I can see this structure (the
[] brackets denote List<TABLE_LIST> bounds):

 
  ['t1'---'(sj-nest)']
               |
            ['alias1'---'t2' ]
               |
             [t3]


'alias1' is on the inner side of an outer join (it has non-NULL on_expr, and outer_join==1). 
It has got a single child, t3, which has on_expr==NULL and outer_join==0.  Is it really correct 
that simplify_joins() didn't remove 'alias1', like it does with regular join nests?

Could it be that simplify_joins() code can't handle single-child join nests just because the parser
never produced them (but after conversion of derived tables to join nests they are now possible?)

BR
 Sergey
-- 
Sergey Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog