Re: RE: query slow in 9i, but not slow in 8i

  • From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
  • To: <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 2 Mar 2004 08:37:12 -0000

Note in-line

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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----- Original Message ----- 
From: "Mark Richard" <mrichard@xxxxxxxxxxxxxxxxx>
>
> I am unfortunately speaking from heresay, however I heard once that having
> histograms all over the place can slow down parsing as they have to be
> inspected.  I was left with the impression that unless the data is skewed
> enough to warrant a histogram then there actually is a negative cost
> associated with having too many histograms.
>

I have to disagree with part of Wolfgang's comment.

To use a histogram, Oracle has to load it into memory,
then compare predicate values with end-points before
producing a selectivity value.

If you have histograms on every single column in the
database, that's a lot of memory to load - and it seems
to be protected by only one latch.  The incremental
CPU cost of using the histogram for any one optimisation
call is probably not significant - but the infrastructure
cost is.

If you have a perfect system, that uses a few distinct
thousand SQL statements, and optimises them just
once, then the overhead is irrelevant.  If you have a
typical system, then it's another nail in the coffin.



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