(8) Composite Key Index
Composite key is a very generic technique, but it is extremely beneficial when a store with ordered keys is used. Composite keys in conjunction with secondary sorting allows one to build a kind of multidimensional index which is fundamentally similar to the previously described Dimensionality Reduction technique. For example, let’s take a set of records where each record is a user statistic. If we are going to aggregate these statistics by a region the user came from, we can use keys in a format (State:City:UserID) that allow us to iterate over records for a particular state or city if that store supports the selection of key ranges by a partial key match (as BigTable-style systems do):
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SELECT Values WHERE state="CA:*"
SELECT Values WHERE city="CA:San Francisco*"
Composite Key Index
Applicability: BigTable-style Databases