I have just started reading about Encoded Vector Index (or EVI)
One of the points that seems to be stressed is cardinality as far as
the key is concerned.
The states of the USA is good cardinality as there are only 50 of them.
Timestamps are bad, because you can have up to a kajillion of them.
How do you determine what is good and what is bad?
For instance, a date field (or at least a numeric field which
represents a date), would that be considered a bad choice for
cardinality.
Cardinality - I hope I have the correct word
I've created them over fields with a much higher number of values. If you
look at the parameters for creation you will see that it can support
65,535 distinct values with a 4 byte code. Just be aware that the number
of possible values allowed will be based on the number of distinct values
in the file, unless you purposely force it to reserve the larger 4 byte
code size. This is one reason it's handy to remove the EVIs before large
updates to the underlying file--you risk exceeding the number of distinct
values for which the EVI was created.
Even if you don't exceed that number, it is still better to remove and add
back the EVIs when doing large updates. For a datamart we build on a
nightly basis, it is MUCH faster to remove the EVIs, rebuild the datamart
(a single, large table), then add back the EVIs than to update the file
while the EVIs are present.
As far as your date field goes, will you have more than 65,535 possible
values? I know what the standard IBM recommendation based on 50 state
codes, but with our little datamart we saw considerable speed improvement
when we added an EVI over the zip codes used in our sales history. Way
more than 50 fields, but still less than the 65k number.
Andrew Lopez
Systems Analyst
Phone: 803-714-2037
Email: ALopez@xxxxxxxxxx
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