How to write SQL hints for this query?

Question

The query is like:
select * from foo, t
where foo.name in
(select name from bar
where id in (
select id from car
where date < sysdate
and foo.a = t.b;
I want the innermostsubquery 'select id from car ...' to be executed first, and the subquery 'select name from bar ...' to be execute second, and the outermost query 'select * from foo,t ...' to be executed the last. How can I write the Oracle sql hints to force the order?
Thanks.

Answer 1

user553560
You might be able to create a large set of hints to force the access path you want - but unless you really know what you are doing with hints, you may find that your solution is very unstable (it might be luck rather than correctness that let's it work to start with).
The difficulty in this query is the double layer of IN subqueries, so if you can rewrite the query, you might try manually unnesting as follows:
select
     t1.*. t.*
from
          select     
               distinct t2.name
          from     t2
          where     t2.id in (
                    select     t3.id
                         from     t3
                    where     t3.dated < sysdate
     )     v,
     t1,
     t
where
     t1.name = v.name
and     t1.a = t.bDepending on your indexing and statistics, you may find that a simple /*+ unnest */ hint in the first subquery will be sufficient to do this for your. Again depending on the statistics you may find that you have to put extra hints into the above to make Oracle use the join method and indexes you think appropriate.
N.B. The first step (as others have noted) is to check that your statistics are good before you start manipulating the code or using hints.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk