Re: Estimate versus Compute

Are you using
a) analyze or dbms_stats
b) block sampling or row sampling

The argument for estimate > 1% causing all blocks to be read anyways goes something like this:
If you are using row sampling with 25% (for the sake of argument) and you have more than 4 rows per block on average then a random sampling of 25% (1 out of every 4 rows) is likely to give you a row from every block. That argument of course doesn't hold if you are using block sampling. Then a 25% sample will read 1/4 of all blocks.

Allen, Brandon wrote:
> Mike, I don't believe that is true. I recently reduced from compute to estimate=25% and this reduced the runtime for update stats from 38 hours to 14 hours. I don't think it would make such a difference if it still had to visit every block. I think you can specify whether your samples are based on rows or blocks and Oracle will automatically compute if you specify estimate >= 50%.
>
> Regards,
> Brandon
>
>

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Regards

Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com
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