Size isn't everything
Why are IT folks obsessed about this? I can recall speaking at a data warehouse conference a few years ago and speaker after speaker eagerly quoted the size of his data warehouse as some sort of badge of courage: "Well, you should see how big mine is...". Of course companies that sell hardware and disk storage love such things, but why is there such a big discrepancy between the behemoths in the Winter Group survey and the less than a terabyte brigade? The answer is quite simple: business to business companies don't have large transaction volumes. If you are a large retailer or a high-street bank, then you may have thousands of branches, each one contributing thousands of individual transactions a day. These add up, and constitute the vast majority of the volume in a data warehouse (perhaps 99% of the volume). The rest of the data is the pesky master data (or reference data, or dimension data - choose your jargon) such as "product", "customer", "location", "brand", "person", "time" etc that provides the context of these business transactions. You may have millions of transactions a day as a retailer, but how many different products do you stock? 80,000 for a convenience store chain? 300,000 for a department store? Certainly not tens of millions. Similarly McDonalds has 27,000 retail outlets, not millions. The same for organizational units, employees etc. One exception that can be very large is "customer" but again this is true only for business to consumer enterprises e.g. retailers or Telcos. Companies like Unilever are very large indeed, but primarily sell to other businesses, so the number of direct customers they deal with is measured in the many thousands, but not millions.
So B2B enterprises usually have quite small data warehouses in volume, even though they may have extremely sophisticated and complex master data e.g. elaborate customer segmentation or product or asset classification. One way to measure such complexity is by adding up the types of business entity in the data model e.g. each level of a product hierarchy might count as one "class of business entity" (CBE), "customer" as another. Some very large data warehouses in volume terms often have very simple business models to support, perhaps with 50 CBEs. On the other hand a marketing system for a company like BP may have 400 or more CBEs. This dimension of complexity is actually just as important as raw transaction size when looking at likely data warehouse performance. A data warehouse with 1TB of data but 50 CBEs may be a lot less demanding than one with 200GB of data but 350 CBEs (just think of all those database joins). Oddly, this complexity measure never seems to feature in league tables of data warehouse size, perhaps because it doesn't sell much disk storage. I feel a new league table coming on. Anyone out there got a model with more than 500 CBEs?