Re: Reverse Key Index

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.

--
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Jonathan Lewis
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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!

>

>

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