There is no correlation between uniqueness and "skewness". For example, you might have unique values: 1,2,3,4,5,...,9999, and then 100000000000000
Obviously that last one is an skew value. If you have predicates on this column along the lines of: where col > ... where col < ... where col between ... and ... then a histogram might be useful, otherwise the optimizer assumes an even distribution of values between 1 and 100000000000000 hth connor --- Murali Menon <[EMAIL PROTECTED]> wrote: > > Uniqueness indicates that there are less skewed > values correct! Under what circumstances would a > skewed column value for a unique column exist. > > Another question, what about histograms on indexes > itself? If histograms are generated on columns with > skewed values, does it matter when the indexes are > generated, before or after? Do indexes on these > columns need to be rebuilt. > > Thanks > > > > > > Jonathan Lewis <[EMAIL PROTECTED]> wrote: > > It's worth mentioning a frequently overlooked > point that even if a column has a unique > constraint (whether or not declared and/or > supported by an index) then if it has a > skewed pattern of values the optimiser > can use a histogram to optimise a queries > that target a range. > > (NB The grammar suggests that the word > "not" was intended as the last word of the > first line of (b) below). > > > Regards > > Jonathan Lewis > http://www.jlcomp.demon.co.uk > > Coming soon one-day tutorials: > Cost Based Optimisation > Trouble-shooting and Tuning > Indexing Strategies > (see http://www.jlcomp.demon.co.uk/tutorial.html ) > > ____UK_______March 19th > ____USA_(FL)_May 2nd > > > Next Seminar dates: > (see http://www.jlcomp.demon.co.uk/seminar.html ) > > ____USA_(CA, TX)_August > > > 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 > Date: 08 February 2003 10:44 > > > > > >b) The number of distinct/duplicate values should > >really be relevant - its whether you will regularly > >need to probe a table using column values that are > >skewed in such a way as to have the optimizer make > >poor assumptions about their distribution. > > > > > -- > Please see the official ORACLE-L FAQ: > http://www.orafaq.net > -- > Author: Jonathan Lewis > INET: [EMAIL PROTECTED] > > 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: [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!? > Yahoo! Mail Plus - Powerful. Affordable. Sign up now ===== Connor McDonald web: http://www.oracledba.co.uk web: http://www.oaktable.net email: [EMAIL PROTECTED] "GIVE a man a fish and he will eat for a day. But TEACH him how to fish, and...he will sit in a boat and drink beer all day" __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com -- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: =?iso-8859-1?q?Connor=20McDonald?= INET: [EMAIL PROTECTED] 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: [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).
