Fw: RE: explain plan and the real execution plan
- From: Kellyn Pedersen <kjped1313@xxxxxxxxx>
- To: oracle Freelists <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 17 Nov 2009 08:32:37 -0800 (PST)
So others can correct me if I'm missing something here- I do love dynamic
sampling, (NOT!)
--- On Tue, 11/17/09, Kellyn Pedersen <kjped1313@xxxxxxxxx> wrote:
From: Kellyn Pedersen <kjped1313@xxxxxxxxx>
Subject: RE: explain plan and the real execution plan
To: lzeng@xxxxxxxxxxxxx
Date: Tuesday, November 17, 2009, 9:30 AM
It could- If the column data was askew across the columns enough for different
samples to cause different execution plans and not statistics on the table and
the query was using literals to the point to cause the CBO to take a dynamic
sample each time. I have seen this, but I've also fixed the stats and made the
developers fix the code...:)
If you seeing this, you could create a SQL Profile, provide hints in the
statement involved, lock down the stats or disable dynamic sampling on the
database if you NEED a more stable execution plan,
(OPTIMIZER_DYNAMIC_SAMPLING=0). I've done this at a previous company where it
involved a web property that NEEDED to stay very static with it's performance.
We were about 99% successful as there are a number of parameters involved in
the CBO doing it's job of finding better execution plans and rewriting
execution plans and queries... :)
Kellyn Pedersen
Multi-Platform DBA
I-Behavior Inc.
http://www.linkedin.com/in/kellynpedersen
"Go away before I replace you with a very small and efficient shell script..."
--- On Mon, 11/16/09, Lei Zeng <lzeng@xxxxxxxxxxxxx> wrote:
From: Lei Zeng <lzeng@xxxxxxxxxxxxx>
Subject: RE: explain plan and the real execution plan
To: "Kellyn Pedersen" <kjped1313@xxxxxxxxx>, vishal@xxxxxxxxxxxxxxx, "Oracle -
L" <oracle-l@xxxxxxxxxxxxx>
Date: Monday, November 16, 2009, 10:45 PM
Would dynamic sampling generates different stats each time if the underlying
table stays static ( no ddl or dml change, no new indexes, ... nothing changes
) ?
Thanks,
Lei
From: Kellyn Pedersen [mailto:kjped1313@xxxxxxxxx]
Sent: Monday, November 16, 2009 7:41 PM
To: vishal@xxxxxxxxxxxxxxx; Oracle - L; Lei Zeng
Subject: RE: explain plan and the real execution plan
Yes- After your explain plan and then upon executing the statement, dynamic
sampling could easily occur and calculate a better execution plan, utilizing it
for the actual execution than it did for the explain plan.
A quick trace of the session would capture the statement, (You are looking for
OPT_DYN statement for one or more tables involved..) or if unable to reproduce,
you could look in the SQL history, either through AWR or through OEM.
Hope this helps,
Kellyn
Kellyn Pedersen
Multi-Platform DBA
I-Behavior Inc.
http://www.linkedin.com/in/kellynpedersen
"Go away before I replace you with a very small and efficient shell script..."
--- On Mon, 11/16/09, Lei Zeng <lzeng@xxxxxxxxxxxxx> wrote:
From: Lei Zeng <lzeng@xxxxxxxxxxxxx>
Subject: RE: explain plan and the real execution plan
To: vishal@xxxxxxxxxxxxxxx, "Oracle - L" <oracle-l@xxxxxxxxxxxxx>
Date: Monday, November 16, 2009, 4:47 PM
Suppose I am using the same session to run both. Could it be different?
Thanks,
Lei
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Vishal Gupta
Sent: Monday, November 16, 2009 3:39 PM
To: Oracle - L
Subject: Fwd: explain plan and the real execution plan
Changes in explain plan and real execution plan depends on the env differences
between the sessions in which you are explain planning and the one in which you
are executing the statement. Sometimes app changes session level parameters at
login time which can change execution plan.
Regards,
Vishal Gupta
On 16 Nov 2009, at 23:28, "Lei Zeng" <lzeng@xxxxxxxxxxxxx> wrote:
Hi, all:
I have a question about the explain plan.
Under what conditions, could the plan I get out of ‘explain plan’ differ from
what I get from running the sql statement?
Suppose the database has no change during I ran ‘explain plan’ and run the sql
– I mean, no ddl and dml – You would think this database is static. Or the time
gap between I run ‘explain plan’ and run sql statement is too narrow to allow
any change (ideal environment). No bind variable peeking because
_optim_peek_user_binds is set to FALSE.
Any idea? Thanks in advance.
Lei
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