Re: USE_HASH hint

I find it interesting that the optimizer seems to be increasingly fussy about hints as time passes.

Once upon a time, it used to be the case that hints could be very sketchy and you would get the result you wanted. Now it seems that your hints have to be very precise, or Oracle may find a path that allows it to ignore your hint.

In a recent example I found that in:

    select /*+ use_nl(t2) *./

       . . .
    from t1, t2
    etc...

the hint was ignored, and a hash join
was used, but if I used the hints:

    /*+ ordered use_nl(t2) */
then the pair of hints was obeyed.

My interpretation was that Oracle examined paths with the order (t2, t1) since I had not STOPPED it from considering different orders, and discovered that a hash join was a good option - then, having decided that my use_nl was 'wrong' with respect to its chosen path it ignored it, but then switched the join inputs to end up with my desired ordering, but
rejected path.

Jonathan Lewis
http://www.jlcomp.demon.co.uk

Host to The Co-Operative Oracle Users' FAQ http://www.jlcomp.demon.co.uk/faq/ind_faq.html

Author of:
Practical Oracle 8i: Building Efficient Databases

Screen saver or Life saver: http://www.ud.com Use spare CPU to assist in cancer research.

-----Original Message-----
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Date: 04 September 2001 22:30

|Thanks for posting the results. I'm endlessly fascinated (and
frustrated)
|by Optimizer behavior.
|
|Jay Miller

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-- 
Author: Jonathan Lewis
  INET: jonathan_at_jlcomp.demon.co.uk

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