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From Enterprise IT Planet]
To improve data query performance, many DBAs implement data caches--summarized and/or specifically extracted data tables that are periodically refreshed from the raw transactional information--that allow BI analysts to receive quicker reponses to their queries than they would if they were forced to go directly to the main transactional DBs themselves. A potential problem with this approach, however, is deciding what data to cache, and then having to pre-define selected queries for the analysts that will run properly from that cached data.
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http://www.enterpriseitplanet.com/networking/features/article.php/3588866