I have a friend who says "always is never true and never is always false"
perhaps we should apply that to all Oracle "certainties"? --- Jonathan Lewis <[EMAIL PROTECTED]> wrote: > > Interesting stuff, but an interesting example of > how myths take off. Did you notice the directive: > > "Rewrite all correlated subqueries using in-line views". > > But you can't guarantee that a rewrite can be > achieved that will produce the same results. > > If you don't believe that, look at the Niemiec > book, page 48, where one of his performance > successes is to turn a join into a correlated > subquery --which in general WILL NOT RETURN > THE SAME RESULT !!! If you doubt that, > think about the case for the 'real-life' example > where an order has two lines for the same value. > > So, what's the bet that two years from now, > you'll be sitting in front of someone saying: > > Yes, I KNOW that Gaja says ... but your > rewrite was wrong, and in this case a > rewrite is not possible. > > > BTW - Oracle 9 will do the transform internally > anyway into the 'corrected' solution, and Oracle 8.1.7 > will do the same if you set > "_unnest_subquery=true". > > > Of course, another argument for being careful > how you state such advice (perhaps "Look for > opportunities to .... because they can ...." is > better than a firm directive) is that there are > always other solutions, such as: > > select > dept_no, > emp_no, > sal > from ( > select > dept_no, > emp_no, > sal, > avg(sal) over(partition by dept_no) as avg_sal > from emp > ) emp_avg > where > sal > avg_sal > ; > > With a test case I set up, the CPU usage was > slightly higher, but the logical I/O and latching > significantly lower using this approach when > compared to the hash-join method. > > > > Jonathan Lewis > http://www.jlcomp.demon.co.uk > > Next Seminar - UK, April 3rd - 5th > http://www.jlcomp.demon.co.uk/seminar.html > > 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 > > > > ----- Original Message ----- > To: Multiple recipients of list ORACLE-L > <mailto:[EMAIL PROTECTED]> > Sent: Wednesday, March 13, 2002 11:28 AM > > > Hey Guys, > > Did anyone have a look at Gaja's new paper on myths and folklore > about > Oracle at Craig Shallahamer's site? Wonderful reading. > > > > -- > Please see the official ORACLE-L FAQ: http://www.orafaq.com > -- > Author: Jonathan Lewis > INET: [EMAIL PROTECTED] > > Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 > San Diego, California -- Public Internet access / Mailing > Lists > -------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (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). __________________________________________________ Do You Yahoo!? Try FREE Yahoo! Mail - the world's greatest free email! http://mail.yahoo.com/ -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Rachel Carmichael INET: [EMAIL PROTECTED] Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (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).
