RE: accessing the records in a history file

["David Morris" <dmorris@xxxxxxxxxxxxx>]

Al & Booth,

Encoded Vector indexes contain more than on-off bits for selected records.  
That is 
what is known as a bit-mapped index.  The confusion is probably that an EVI is 
often 
used to improve the build time of a bit-mapped index.  An EVI contains two 
arrays.  
One is a table containing key values, an integer code (the vector), a count for 
the 
key value, and the first and last row containing the value.  The other table 
contains 
the assigned codes (vectors).  The main advantage of this is that the optimizer 
has 
really good information to work from.  Rather than seeing "=" and assuming that 
this 
will include 20% of the records in a file (I am not sure if it is actually 
20%), the optimizer 
can optimize for actual number of records that will be selected. 

I have created quite a few of these when optimizing database selections and 
have yet 
to see a 95% improvement, but 10-20% is common.

David Morris

>>> barsa2@ibm.net 12/21/00 12:44PM >>>
At 07:18 PM 12/21/00 +0000, you wrote:
>What is an Encoded Vector Index?  This is a new term for me.


It's a new type of index, added in a recent release (I think 4.2 otherwise 
4.4), where an index with select omit criteria is created in memory with 
one bit (on or off) signifying that a record is to be selected or 
omitted.  As you typically can get that much memory available, this is 
tantamount to putting the select omit portion of the index totally into 
memory.  If you use SQL, and if you can use this feature, you can drop the 
query portion of time by up to 95%.  (Available only in SQL, not OPNQRYF or 
DDS, because someone in Rochester doesn't give a @#$% about existing 
users.)  An AS/400 first!

Al


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