Re: explain plan and the real execution plan
- From: Tanel Poder <tanel@xxxxxxxxxx>
- To: lzeng@xxxxxxxxxxxxx
- Date: Tue, 17 Nov 2009 08:54:27 +0800
3 main reasons:
1) The optimizer statistics the EXPLAIN PLAN ends up using are different
from the statistics the other session ended up using
2) Explain plan does not use bind variable peeking thus will not optimize
for current bind variable values
3) Explain plan treats all bind variables as VARCHAR2, thus you ma have
implicit datatype conversion happening during the plan execution, (meaning
to_char,to_number functions are added around variables/columns) and this for
example may make optimizer to ignore some indexes if you get unlucky.
Tanel Poder
http://blog.tanelpoder.com
On Tue, Nov 17, 2009 at 7:25 AM, 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
>
>
>
--
Tanel Poder
http://blog.tanelpoder.com
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