RE: bind vars change explain plan

I think I saw the correct answer to this one shoot by on the list. In Oracle8, the use of bind variables completely prevents the Oracle query optimizer from using histograms. Could make a big difference in which plan CBO selects if there's a lot of data skew.

In Oracle9, it gets a little better, but things still aren't completely right. Apparently, a session only peeks at the value once and then uses the first plan it computes thereafter.

Barb's bottom line is absolutely correct; use bind variables. However you can.

Cary Millsap
Hotsos Enterprises, Ltd.
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-----Original Message-----
Barbara
Sent: Monday, July 22, 2002 10:08 AM
To: Multiple recipients of list ORACLE-L

prem.
Never did get an answer to this question. I don't know why using bind variables changed the execution path. My best guess comes from the developer. She thinks that when we supplied the values, the optimizer knew
what the range of values would be, and could therefore determine to use the
index. With the bind variable, the optimizer did not have a range of values
to work with and therefore did not choose the index in the execution path.

I have no knowledge that using bind variables will suppress indexes. Just
happened that it did in this case.
Also keep in mind that this particular database is using an old version of
Oracle (7.3.4). Optimizer got much better in version 8.

The list helped me out with a work-around, which was to index-hint the index
I wanted.

Bind variables are definitely "good guys". I highly recommend you continue
with your code changes to include binds.

Good luck.
Barb

> ----------
> From: oraora oraora[SMTP:oraoraora_at_rediffmail.com]
> Reply To: oraora oraora
> Sent: Sunday, July 21, 2002 8:24 PM
> To: bbaker_at_denvernewspaperagency.com
> Subject: Re: bind vars change explain plan
>
> Baker,
>
> sorry i did not read the reply to ur query.
> what was the reply ?
> will using bind vars suppress index ?
> kindly let me know b'coz i have also changed my code to SQL with
> bind vars just now.
>
> Regards,
> prem.
>

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