what is a junk dimension ?

Answer Posted / vijay

A "junk" dimension is a collection of random transactional
codes, flags and/or text attributes that are unrelated to
any particular dimension.

The junk dimension is simply a structure that provides a
convenient place to store the junk attributes. A good
example would be a trade fact in a company that brokers
equity trades.

The fact would contain several metrics (principal amount,
net amount, price per share, commission, margin amount,
etc.) and would be related to several dimensions such as
account, date, rep, office, exchange, etc. This fact would
also contain several codes and flags that were related to
the transaction rather than any of the dimensions ... such
as origin code (that indicates whether the trade was
initiated with a phone call or via the Web), a reinvest
flag (that indicates whether or not this trade as was the
result of the reinvestment of a dividend payout) and a
comment field for storing special instructions from the
customer.

These three attributes would normally be
removed from the fact table and stored in a junk
dimension ... perhaps called the trade dimension. In this
way, the number of indexes on the fact table would be
reduced, and performance (not to mention ease of use) would
be enhanced. Hope this helps.

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