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 <[EMAIL PROTECTED]>
Date: 04 September 2001 22:30
|Thanks for posting the results. I'm endlessly fascinated (and
frustrated)
|by Optimizer behavior.
|
|Jay Miller
--
Please see the official ORACLE-L FAQ: http://www.orafaq.com
--
Author: Jonathan Lewis
INET: [EMAIL PROTECTED]
Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
San Diego, California -- Public Internet access / Mailing Lists
--------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from). You may
also send the HELP command for other information (like subscribing).
