query slow in 9i, but not slow in 8i 2004-03-02 - By Jonathan Lewis
Note in-line
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
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-- -- Original Message -- --
From: "Mark Richard " <mrichard@(protected) >
>
> 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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