Re: Reverse Key Index

Jonathan Lewis wrote:

> Don't try to understand it - it's wrong.
>
> "Balanced B-tree" indexes as used by
> Oracle are always 'balanced' - it's part
> of the definition. This means that all LEAF
> blocks are the same distance from the ROOT
> block at any moment in time.
>
> The comments you are looking at probably
> relate to the fact leaf blocks split when full
> and different arrival patterns for new data
> can result in different degrees of wastage
> of space (on average) in a block because of
> the long-term effects of splitting.
>
> In theory the leaf blocks of a balanced b-tree
> will operate, on average, over large volumes of
> data arriving in random order, with 25% wastage
>
> There is a well-known myth that sequentially
> arriving data will cause the index to operate
> at 50%.wastage. In fact it will operate at close
> to 100% if the data really does arrive sequentially.
>
> However, if the column is generated by a sequence
> number, but the application coding strategy stops
> the data from arriving sequentially, you can pick any
> number between 0% and 50% as the amount of
> wastage in the block.
>
> Using a reverse key index tends to flatten out the
> wastage (to the standard 25%) by "randomising"
> the stored value.
>

It makes sense. Thank you, Jonathan!

>
> --
> Regards
>
> Jonathan Lewis
> http://www.jlcomp.demon.co.uk
>
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>
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>
> y wrote in message <3D93A7E7.E9E9B35B_at_y.com>...
> >Hi,
> >Suppose I hava a table which contains sequential numbers. And I need
> >build an index on that column. I checked the manual, still can not
> >understand why RKI(Reverse Key Index) can build more evenly distributed
> >index tree than a B-tree.
> >Any help would be appreciated!
> >
> >
> >
> >
Received on Fri Sep 27 2002 - 07:23:46 CDT