RE: Index help

From: Powell, Mark D <mark.powell_at_eds.com>
Date: Mon, 26 Jul 2004 11:35:37 -0400
Message-ID: <564DE4477544D411AD2C00508BDF0B6A2133DE2C@usahm018.exmi01.exch.eds.com>

If we go back to the original post with the Query then there are 3 columns referenced in the WHERE clause. It may still be possible to improve the query by building an index on those other columns. So far all the discussion has been on IDA3A5 which because it is badly skewed is not a good overall index choice.

HTH -- Mark D Powell --

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of Daniel Fink Sent: Monday, July 26, 2004 11:08 AM
To: oracle-l_at_freelists.org
Subject: Re: Index help

If the value of :1 = 46847 a full table scan makes sense (96% of the rows meet the predicate condition). If the value is not 46847, then an index lookup is probably faster. This is really a catch-22. Bind variables are helping your parse times, but *may* be hurting your execution time (hard to tell without knowing how many executions use 46847 and how many do not). Without a histogram on the column, Oracle will assume an even distribution, which is incorrect.

You are trapped between a rock and a hard place. If you use hard-coded values instead of binds and have a histogram on the column, the CBO should pick the proper execution plan each time. This also means that you will have unique versions of the statement in your shared pool. Sorry there is not a simple, quick answer to this one.

I suggest reading Wolfgang's paper "Fallacies of the Cost-Based Optimizer" at www.centrexcc.com. He explains this issue very well.

Regards,
Daniel Fink

M.Godlewski wrote:
> Data distribution.
>
> IDA3A5 count(IDA3A5)
> 4104 44
> 4107 156
> 4110 1
> 4111 1
> 4137 1
> 46847 4905

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Received on Mon Jul 26 2004 - 10:32:36 CDT