Re: Encoded Vector Indexes -EVI's

["R. Bruce Hoffman, Jr." <rbruceh@xxxxxxxxxxxxx>]

-----Original Message-----
From: John P Carr <jpcarr@tredegar.com>
To: MIDRANGE-L <MIDRANGE-L@midrange.com>
Date: Tuesday, February 29, 2000 10:01 AM
Subject: Re: Encoded Vector Indexes -EVI's


>
>It is not however a  " Everywhere,  Everytime, for Everybody " type of
>thing.
>They should be used where there is a relatively small number of distinct
>values for example.
>


uuuuhhhhhhh....

High cardinality, not necessarily low cardinality. The range is controllable
with the default value of 256 distinct values. The actual number of distinct
values (cardinality) is used to build the index and the value is used to
determine whether the encoding should be stored in 2 bytes, 4 bytes or 8
bytes. The use of the EVI allows faster selection based on selection of the
field. In the degenerative case of 1 value for all records in a file, the
EVI (or even a regular index for that matter) provide no refinement or
partitioning of the table based on the predicate and thus a table scan
occurs without any performance improvement from indexing.



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