(Re): Really Wierd Query tuning issue

From: Wolfgang Breitling <breitliw_at_centrexcc.com>
Date: Thu, 12 Aug 2004 20:33:52 -0600
Message-Id: <6.1.0.6.2.20040812202932.032fb308@pop.centrexcc.com>

What's your db_file_multiblock_read_count set at, or are you using system stats? From the absence of cpu costs in the explain plan I'd say no.

Without the index access, the order by requires a sort, which could put the cost of a "scan + sort" higher than the index access by unique index. A 10053 trace might be interesting and may help explain things, provided the two plans ARE actually different.

At 08:10 PM 8/12/2004, you wrote:
>I thought
>
>'set autotrace on' was already correct? It was just 'explain plan for' that
>could be inaccurate?
>I'll check the trace tomorrow. I think you are right about the full table
>scan vs. the index scan.
>
>I did a unique scan and retrieved 1 record. The index is 1 block. So it
>should be two LIOs. One to read the index block and 1 to read the record in
>the table. Wierd that an 'order by' would coax oracle to do the right plan.
>
>The table fits inside one 1 MB extent. Any idea how to calculate how many
>expected LIOs for a full table scan? If I do:
>1 MB / 8192 block size I get 125, which is just about double my 63 LIOs?

Regards

Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com

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Received on Thu Aug 12 2004 - 21:29:36 CDT