RE: What else can cause different optimizer plan?

  • From: "Iotzov, Iordan" <IIotzov@xxxxxxxxxxxxxxx>
  • To: "paul.baumgartel@xxxxxxx" <paul.baumgartel@xxxxxxx>, "kerry.osborne@xxxxxxxxxxx" <kerry.osborne@xxxxxxxxxxx>, "cshapi@xxxxxxxxx" <cshapi@xxxxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
  • Date: Thu, 8 Dec 2011 16:58:54 -0500

You can run event 10053 trace for each situation 
( - it shows the 
calculations the CBO performs to get the execution plan. Look at the two trace 
files and see where they start to diverge. That may give you the clue about why 
the CBO generates different execution plan.

Iordan Iotzov

-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On 
Behalf Of paul.baumgartel@xxxxxxx
Sent: Thursday, December 08, 2011 4:13 PM
To: kerry.osborne@xxxxxxxxxxx; cshapi@xxxxxxxxx; oracle-l@xxxxxxxxxxxxx
Subject: RE: What else can cause different optimizer plan?

Thanks for your replies.  A few observations in response to the points

 optimizer_env_hash_value is the same for both--does this not mean that
optimizer environments are identical?
 Underlying objects are the same (it's the same query issued by the same
 Query in pipelined function has one variable, which is passed in as
parameter.  Both sessions pass the same value.
 Plans displayed by dbms_xplan do not show use of cardinality feedback.
 No outlines, profiles, or baselines exist.
 No PX in either case.
 No histograms.

Paul Baumgartel
Group Finance Accounting Solutions
400 Atlantic Street
Stamford, CT 06904



-----Original Message-----
From: Kerry Osborne [mailto:kerry.osborne@xxxxxxxxxxx]
Sent: Thursday, December 08, 2011 3:21 PM
To: cshapi@xxxxxxxxx
Cc: Baumgartel, Paul; oracle-l@xxxxxxxxxxxxx
Subject: Re: What else can cause different optimizer plan?

It's a long list. Here are a few more to come readily to mind.

Different optimizer environment (i.e. any of a whole bunch of parameters
is different)
Different underlying objects (your MYDATA table is not the same as Joe's
MYDATA table)
Data changes (range of value significantly exceeds what stats says it
Bind Variable Peeking kicks in
Cardinality Feedback feature kicks in
Adaptive Cursor Sharing kicks in
Someone creates an Outline, Profile, or Baseline on the statement
Size of memory structures (can determine whether a statement gets run
with PX plan or not)

The most common cause of plan instability is probably the combination of
histograms and bind variables (i.e. bind variable peeking).

Kerry Osborne

On Dec 8, 2011, at 2:04 PM, Gwen Shapira wrote:

> different statistics on objects, different system statistics or
> different bind variables.
> Gwen
> On Thu, Dec 8, 2011 at 10:39 AM,  <paul.baumgartel@xxxxxxx> wrote:
>> --
>> //
> --
> //

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