Re: Plea for Query Tuning Help

From: Charlotte Hammond <charlottejanehammond_at_yahoo.com>
Date: Tue, 12 Sep 2006 13:31:15 -0700 (PDT)
Message-ID: <20060912203115.37257.qmail@web34115.mail.mud.yahoo.com>

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_at_yahoo.com> 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.
> >
> >

---

Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

--
http://www.freelists.org/webpage/oracle-l

Received on Tue Sep 12 2006 - 15:31:15 CDT