Re: RE: query slow in 9i, but not slow in 8i

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Tue, 2 Mar 2004 08:37:12 -0000
Message-ID: <008501c40031$911d23a0$6702a8c0@Primary>

Note in-line

Regards

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

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

March 2004 Hotsos Symposium - The Burden of Proof   Dynamic Sampling - an investigation
March 2004 Charlotte OUG (www.cltoug.org) CBO Tutorial April 2004 Iceland
June 2004 UK - Optimising Oracle Seminar

• Original Message ----- From: "Mark Richard" <mrichard_at_transurban.com.au>
  >
  > I am unfortunately speaking from heresay, however I heard once that having
  > histograms all over the place can slow down parsing as they have to be
  > inspected. I was left with the impression that unless the data is skewed
  > enough to warrant a histogram then there actually is a negative cost
  > associated with having too many histograms.
  >
  

I have to disagree with part of Wolfgang's comment.

To use a histogram, Oracle has to load it into memory, then compare predicate values with end-points before producing a selectivity value.

If you have histograms on every single column in the database, that's a lot of memory to load - and it seems to be protected by only one latch. The incremental CPU cost of using the histogram for any one optimisation call is probably not significant - but the infrastructure cost is.

If you have a perfect system, that uses a few distinct thousand SQL statements, and optimises them just once, then the overhead is irrelevant. If you have a typical system, then it's another nail in the coffin.

---

Please see the official ORACLE-L FAQ: http://www.orafaq.com

---

To unsubscribe send email to: oracle-l-request_at_freelists.org put 'unsubscribe' in the subject line.

--
Archives are at http://www.freelists.org/archives/oracle-l/
FAQ is at http://www.freelists.org/help/fom-serve/cache/1.html
-----------------------------------------------------------------

Received on Tue Mar 02 2004 - 02:34:08 CST