Larry's answer is totally correct -
The SQL reads:
select /*+ use_nl(t2) *./
etc
from t1,t2
where t1.id = t2.id
etc
But manages to hash from T1 to T2.
(I had to play around a little bit to
make the tables and rows the right
size for this to happen).
Looking at the 10053 trace, I got the lines:
(with lots of cuts and editing)
General Join Order:
Join Order 1 T1 -> T2
use_nl
calculations - cost = 92
NO LINES for sort/merge
NO LINES for hash join
so the hint IS constraining Oracle to
consider only NL when joining T2.
Join Order 2 T2 -> T1
Use_nl
calculations - cost = 127
sort/merge
calculations - cost = 64
hash join
calculations - cost = 47
(sides swapped)
So Oracle arrives at the best path by
considering the join order T2 -> T1,
which makes the hint ignorable, but
then swaps the table order at execution
time, to produce the apparent contradiction.
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: 14 September 2001 10:37
|
|thanks for the explanation. I'm interesting in seeing your answer.
|It seems Oracle should rename USE_NL into USE_NL_or_HASH :-)
|
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Author: Jonathan Lewis
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