Newbie question : why is star schema fast and efficient?

Question

Hi all,
just a stupid question, but I haven't been able to find a proper
answer so far...
Why is star schema a good design for Data Marts and DWH?
What is the underlying reason that makes it attractive
performance wise?
Why wouldn't just one big table with all the data in it and with
the proper indexes be enough?
Thanks all!!
Regards
Vincent

Answer 1

There are several reasons to use star schemas, particularly in
Oracle.
A flat table like you asked about looks attractive but has
several flaws, i.e. massive data redundancy, no logical
groupings, no aggregation (or additional redundant data
aggregated), etc.
A start schema is semi-denormalized to allow easy reporting. A
truely normalized system is diffucult to report against be cause
you may have to join many tables to return just 2 pieces of
related data. A star schema enables you to join to only a single
dimension table to the fact table to return the same 2 pieces of
data. If you're returning many pieces of data, a star schema
keeps access very simple. Most third party reporting tools
recognize star schemas and will build your where clauses behind
the scenes making them a lot more useful to end users.
Oracle is adding optimizations to the cbo for start schemas.
Using dimensions, materialized views, partitions, IOTs, etc
greatly enhances performance for queries against massive amounts
of data. It does make loading the data more difficult but the
trade off at query time is worth it.
A flat table structure, besides having a lot of redundant data,
is hard to optimize. When you have terebytes of data, a flat
table structure gets scary even with indexes.
This is just my opinion, hope that helps.
Lewis