Wednesday Mar 09, 2016

OUGN Conference - On the boat again

OUGN Spring Conference 2016

Last year influenza took me down and out just a couple of days before my planned departure for the famous OUGN Spring Conference. But this year (so far) I'm still happy and healthy and on my way towards beautiful Oslo. I'm really looking forward to this year's OUGN Spring Conference which will happen again on the boat departing from Oslo and sailing over to Kiel - and then returning back.

In case you plan to visit my talks and demos:

  • Thursday, 10-March-2016 - 14:00-14:45h - Parliament 1+2
    How Oracle Single Tenant will change a DBA's life
    .
  • Friday, 11-March-2016 - 10:30-11:15h - Parliament 1+2
    Oracle Database Upgrade: Live and Uncensored
    .

Looking forward to this wonderful event with so many good talks and presentations and such a great group of people. And thanks to the organizers of OUGN!

--Mike

.

Tuesday Jul 28, 2015

Optimizer Issue in Oracle 12.0.1.2: "Reduce Group By"

Wrong Query Results BugDBAs biggest fears I'd guess are Optimizer Wrong Query Results bugs as usually the optimizer does not write a message into the alert.log saying "Sorry, I was in a bad mood today ..."

The Oracle Database Optimizer is a complex piece - and in Oracle 12c it delivers great performance results. Plus (my personal experience when you know what to do) it is more predictable which I like a lot when changing databases from one to another release. But due to its complexity sometimes we see issues - and sometimes it is necessary to switch off tiny little pieces until a fix is available.

Roy just came across this one - and we believe it's worth to tell you about it. Again, our intention is only to prevent issues when upgrading or migrating to Oracle Database 12.1.0.2.

Symptom:

An outer join query with a bind variable and a group by clause can produce wrong results in some cases.

Analysis:

 If all of the following match, you may be hitting this bug:
 - two or more subquery views are outer-joined on column C1
 - column C1 is specified on select list of top-most query block
 - column C1 is filtered on a bind value

Example:

 create table test1(c1 number(5),c2 varchar2(16));
 insert into test1 values(1,'3');
 commit;

 set NULL NULL
 variable num1 number
 execute :num1 :=1;

 -- Following query retuns wrong result(NULL), this should return 1.

 select V.c1 from
  (SELECT c1 FROM test1 GROUP BY c1) V,
  (SELECT c1 FROM test1 WHERE c2 = '1' GROUP BY c1) V2
 where  V.c1 = :num1
    and V.c1 = V2.c1(+);

Workaround:

alter session set "_optimizer_reduce_groupby_key" = false;

Please don't use the workaround:
alter session set optimizer_features_enable='12.1.0.1';
as this will switch off other good 12.1.0.2 optimizer features working very well.

More information:

See MOS Note:20634449.8 describing:
Bug 20634449 - Wrong results from OUTER JOIN with a bind variable and a GROUP BY clause in 12.1.0.2

As far as I can see there are no interim (one-off/single) patches available right now. 

--Mike

Thursday Nov 17, 2011

DOAG Conference 2011: Seven Flavors of Database Upgrades

Thanks to everybody who did attend at my DOAG Conference session in Nürnberg this year "Seven Flavor of Database Upgrades" (or in German: "7 Wege zum Datenbank-Upgrade - Geschichten, die das Leben schrieb"). And thanks for your patience staying with me in overtime as well ;-)

In case you'd like to download the slides I've presented at the session please download them via this link or from the download section to your right.

About

Mike Dietrich - Oracle Mike Dietrich
Master Product Manager - Database Upgrade & Migrations - Oracle

Based in Germany. Interlink between customers/partners and the Upgrade Development. Running workshops between Arctic and Antartica. Assisting customers in their reference projects onsite and remotely. Connect via:

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