Re: OCP Question (Perf Tuning)

Note in-line.

Regards

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

  The educated person is not the person
  who can answer the questions, but the
  person who can question the answers -- T. Schick Jr

One-day tutorials:
http://www.jlcomp.demon.co.uk/tutorial.html

Three-day seminar:
see http://www.jlcomp.demon.co.uk/seminar.html ____UK___November

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

• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Tuesday, January 06, 2004 6:59 PM

> Nope. The answer is b). In the FIRST_ROWS mode, optimizer prefers NL to
all other
> methos despite the price.
>

Unless the alternative is a full tablescan on the inner table - in which case merge or hash joins can be considered.

The question itself is non-trivial, as the cost of a nested loop is:

    Cost of outer acquisition +
    Cost of inner access * cardinality of outer acquisition.

But the cost of a merge join is:

    Cost of first acquisition + cost of first sort +     Cost of second acquisition + cost of second sort +     Cost of merge

It seems likely that if the first table returned 1 or 2 rows, then a nested loop with FTS could be cheaper than a sort merge, but if the outer table returned 3 Oracle would switch to a sort merge. (Assuming equijoin).

On the other hand, if the second table required a very large sort, I'm sure you could engineer a sort_area_size that would make the sort cost more than three times the cost of a simple tablescan - which means you could change the access path by changing the sort_area_size.

-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Jonathan Lewis
  INET: jonathan_at_jlcomp.demon.co.uk

Fat City Network Services    -- 858-538-5051 http://www.fatcity.com
San Diego, California        -- Mailing list and web hosting services
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (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).