generic question: update statistics low drop distr

Hi, why should anyone run update statistics low drop distributions ? (maybe except after database upgrades - and even there it is not possible if the database has serveral terabytes) What is the disadvantage of running only NEW update statistics commands: update statistics low .. update statistics medium .. update statistics high (for leading index columns) Regards, Andreas Kutsche ------------------------------------------- SPAR Osterreichische Warenhandels-AG Hauptzentrale A - 5015 Salzburg, Europastrasse 3 FN 34170 a Tel: +43 662 4470 24223 Mobile: +43 664 6259575 E-Mail: Andreas.KUTSCHE@spar.at Internet: http://www.spar.at Wichtiger Hinweis: Der Inhalt dieser E-Mail kann vertrauliche und rechtlich geschutzte Informationen, insbesondere Betriebs- oder Geschaftsgeheimnisse, enthalten, zu deren Geheimhaltung der Empfanger verpflichtet ist. Die Informationen in dieser E-Mail sind ausschlie?lich fur den Adressaten bestimmt. Sollten Sie die E-Mail irrtumlich erhalten haben so ersuchen wir Sie, die Nachricht von Ihrem System zu loschen und sich mit uns in Verbindung zu setzen. Uber das Internet versandte E-Mails konnen leicht manipuliert oder unter fremdem Namen erstellt werden. Daher schlie?en wir die rechtliche Verbindlichkeit der in dieser Nachricht enthaltenen Informationen aus. Der Inhalt der E-Mail ist nur rechtsverbindlich, wenn er von uns schriftlich bestatigt und gezeichnet wird. Sollte trotz der von uns verwendeten Virus-Schutzprogramme durch die Zusendung von E-Mails ein Virus in Ihre Systeme gelangen, haften wir nicht fur evtl. hieraus entstehende Schaden. Wir danken fur Ihr Verstandnis. Important notice: The contents of this e-mail may contain confidential and legally protected information that is in particular related to operational and trade secrets, which the recipient is obliged to treat as confidential. The information in this e-mail is made available exclusively for use by the addressee. In the event that the e-mail may have been sent to you in error, we would ask you to kindly delete this communication from your system and to contact us. E-mails sent via the Internet can be easily manipulated or sent out under someone else's name. We therefore do not accept legal liability for the information contained in this communication. The contents of the e-mail are only legally binding if they have been confirmed and signed by us in writing. If, in spite of our using Antivirus protection software, a virus may have penetrated your system through the sending of this e-mail, we do not accept liability for any damage that may possibly arise as a result of this. We trust that you appreciate our position. -------------------------------------------

Andreas.KUTSCHE@spar.at wrote: > Hi, > > why should anyone run > update statistics low drop distributions ? > (maybe except after database upgrades - and even there it is not > possible if the database has serveral terabytes)> Even on a large database you could run this table by table with far less runtime required especially if you run several tables in parallel. That aside, there are few situations requiring one to drop distributions on a table, besides following a version upgrade. One is if the table is initially nearly empty and is being loaded and must be queried without time in between to create new distributions. Another is a load job that may be adding large numbers of rows to a table which would skew the actual distribution versus the saved distributions and may need to also update large numbers of existing rows. Such a job is far more likely to use a much less than optimal query plan for the updates with distributions than without. Finally there are some queries and physical distributions of keys in a table or tables which will always confuse the optimizer. Other similar scenarios come to mind. Dropping distributions in these situations will trigger the older OL5 style optimizer code which relies only on the stats counts in systables, syscolumns, and sysindexes/sysindices to make reasonable decisions about query plans and is sometimes a better choice. I've sometimes wished we had an OPTIMIZER HINT to disable the cost based optimizer and revert to the older one explicitely. > What is the disadvantage of running only NEW update statistics commands: > update statistics low .. > update statistics medium .. > update statistics high (for leading index columns)> > Regards, > Andreas Kutsche > Art S. Kagel Oninit, LLC <SNIP>

I thought I remember JM saying that US-medium on a table will drop distributions and rebuild them anyways? Bob Roussey Unix / Informix Administration Spirit Airlines Robert.Roussey@SpiritAir.com -----Original Message----- From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of Art S. Kagel (Oninit LLC) Sent: Friday, November 30, 2007 9:52 AM To: ids@iiug.org Subject: Re: generic question: update statistics low dr.... [10554] Andreas.KUTSCHE@spar.at wrote: > Hi, > > why should anyone run > update statistics low drop distributions ? > (maybe except after database upgrades - and even there it is not > possible if the database has serveral terabytes)> Even on a large database you could run this table by table with far less runtime required especially if you run several tables in parallel. That aside, there are few situations requiring one to drop distributions on a table, besides following a version upgrade. One is if the table is initially nearly empty and is being loaded and must be queried without time in between to create new distributions. Another is a load job that may be adding large numbers of rows to a table which would skew the actual distribution versus the saved distributions and may need to also update large numbers of existing rows. Such a job is far more likely to use a much less than optimal query plan for the updates with distributions than without. Finally there are some queries and physical distributions of keys in a table or tables which will always confuse the optimizer. Other similar scenarios come to mind. Dropping distributions in these situations will trigger the older OL5 style optimizer code which relies only on the stats counts in systables, syscolumns, and sysindexes/sysindices to make reasonable decisions about query plans and is sometimes a better choice. I've sometimes wished we had an OPTIMIZER HINT to disable the cost based optimizer and revert to the older one explicitely. > What is the disadvantage of running only NEW update statistics commands: > update statistics low .. > update statistics medium .. > update statistics high (for leading index columns)> > Regards, > Andreas Kutsche > Art S. Kagel Oninit, LLC <SNIP> ************************************************************************ ******* Forum Note: Use "Reply" to post a response in the discussion forum.

Robert Roussey(IT) wrote: > I thought I remember JM saying that US-medium on a table will drop > distributions and rebuild them anyways? > Well, almost. Medium and HIGH both drop the existing distributions and insert new ones in a single transaction. If the process is interrupted the original distributions are still intact. As I said, you really only have to explicitely DROP DISTRIBUTIONS when upgrading to make sure that ALL of the old distributions are gone and if you do not want ANY distributions for an object. Art S. Kagel Oninit, LLC > Bob Roussey > Unix / Informix Administration > Spirit Airlines > Robert.Roussey@SpiritAir.com > > -----Original Message----- > From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of > Art S. Kagel (Oninit LLC) > Sent: Friday, November 30, 2007 9:52 AM > To: ids@iiug.org > Subject: Re: generic question: update statistics low dr.... [10554] > > Andreas.KUTSCHE@spar.at wrote: > >> Hi, >> >> why should anyone run >> update statistics low drop distributions ? >> (maybe except after database upgrades - and even there it is not >> possible if the database has serveral terabytes)>> >> > > Even on a large database you could run this table by table with far less > > runtime required especially if you run several tables in parallel. > > That aside, there are few situations requiring one to drop distributions > > on a table, besides following a version upgrade. One is if the table is > initially nearly empty and is being loaded and must be queried without > time in between to create new distributions. Another is a load job that > may be adding large numbers of rows to a table which would skew the > actual distribution versus the saved distributions and may need to also > update large numbers of existing rows. Such a job is far more likely to > use a much less than optimal query plan for the updates with > distributions than without. Finally there are some queries and physical > distributions of keys in a table or tables which will always confuse the > > optimizer. Other similar scenarios come to mind. > > Dropping distributions in these situations will trigger the older OL5 > style optimizer code which relies only on the stats counts in systables, > > syscolumns, and sysindexes/sysindices to make reasonable decisions about > > query plans and is sometimes a better choice. I've sometimes wished we > had an OPTIMIZER HINT to disable the cost based optimizer and revert to > the older one explicitely. > > >> What is the disadvantage of running only NEW update statistics >> > commands: > >> update statistics low .. >> update statistics medium .. >> update statistics high (for leading index columns)>> >> Regards, >> Andreas Kutsche >> >> > Art S. Kagel > Oninit, LLC > <SNIP> > > ************************************************************************ > ******* > Forum Note: Use "Reply" to post a response in the discussion forum. > > > ******************************************************************************* > Forum Note: Use "Reply" to post a response in the discussion forum. > >