Re: explain plan and the real execution plan

Answers below

>
> 1) The optimizer statistics
>
> I assume this database has no change – no ddl and dml. Under this
> situation, I would say the optimizer statistics would remain the same,
> correct?
>
>

Even if you have no DDL and no DML happening, someone may still have gathered optimizer statistics on the tables and changed them. Maybe the last time stats were gathered was months or years ago (in theory gathering stats should make things better, but in practice its not always so, especially if the new stats aren't collected appropriately).

>
>
> 2) The implicit data type conversion
>
> Would you provide a testing case on this? I tried some queries but I could
> not produce the effects.
>

I wrote an example in my blog:

http://blog.tanelpoder.com/2009/11/17/explain-plan-for-command-may-show-you-the-wrong-execution-plan-part-1/

Tanel Poder
http://blog.tanelpoder.com

--
http://www.freelists.org/webpage/oracle-l