Re: Plea for Query Tuning Help
- From: Charlotte Hammond <charlottejanehammond@xxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Tue, 12 Sep 2006 13:31:15 -0700 (PDT)
Hi All,
Thanks to all who have responded both on and off the
list. A couple of people have asked me to post the
plan but it is huge (due to NASTY_VIEW) so I'd rather
not (at least yet!) as it'd probably be more
off-putting than anything else :-)
Essentially I just want the WHERE id = on NASTY_VIEW
to treat the right value the same if it's a bind or if
it's coming from a sub-query. Many good suggestions
but no luck yet. I thought I'd post the results to
see if anyone can spot anything from them...
Here's an even simpler example:
The Good case again:
SELECT *
FROM NASTY_VIEW
WHERE id = :bind
--> < 1 sec, (CBO=410)
Any now some attempts to use a value from a SELECT.
1. Simple select:
SELECT *
FROM NASTY_VIEW
WHERE id = (SELECT :bind FROM dual)
--> long time (CBO=17378)
2. Try with a join instead:
SELECT *
FROM NASTY_VIEW nv, (SELECT :bind id FROM dual) b
WHERE nv.id = b.id
--> long time (CBO=32731)
3. Try putting in a row number limitation /
pseudo-column
SELECT *
FROM NASTY_VIEW nv, (SELECT :bind id, rownum rn FROM
dual WHERE rownum=1) b
WHERE nv.id = b.id
--> long time (CBO=17388)
4. Try NO_MERGE hint
SELECT *
FROM NASTY_VIEW
WHERE id = (SELECT /*+ NO_MERGE */ :bind FROM dual)
--> long time (CBO=17378) (Same plan as 1)
(Same result if NO_MERGE hint in the outer SELECT)
5. Try FIRST_ROWS(n) hint
SELECT /*+ FIRST_ROWS(1) */
FROM NASTY_VIEW
WHERE id = (SELECT :bind FROM dual)
--> long time (CBO=6030) (Best yet but still very slow
compared to good case; other values of n were the same
or worse)
6. Try hidden parameter tweaking
ALTER SESSION SET "_UNNEST_SUBQUERY"=FALSE; --> same
as 1
ALTER SESSION SET "_COMPLEX_VIEW_MERGING"=FALSE -->
same as 1
Many thanks again for all your time.
Charlotte
> On 9/12/06, Charlotte Hammond
> <charlottejanehammond@xxxxxxxxx> wrote:
> >
> > How can I prevent this happening so that it
> "looks"
> > like the first "fantastic" query? I've tried
> messing
> > with hints (NO_MERGE and PUSH_SUBQ) but I don't
> know
> > enough to do so effectively. Statistics are full
> and
> > up to date. This is Oracle 9.2.0.6.
> >
> >
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