MOLAP Servers: These servers directly support the
multidimensional view of data through a
multidimensional storage engine. This makes it possible
to implement front-end multidimensional queries on the
storage layer through direct mapping. An example of
such a server is Essbase (Arbor). Such an approach has
the advantage of excellent indexing properties, but
provides poor storage utilization, especially when the
data set is sparse. Many MOLAP servers adopt a 2-level
storage representation to adapt to sparse data sets and
use compression extensively. In the two-level storage
representation, a set of one or two dimensional subarrays
that are likely to be dense are identified, through the use
of design tools or by user input, and are represented in
the array format. Then, the traditional indexing structure
is used to index onto these “smaller” arrays. Many of the
techniques that were devised for statistical databases
appear to be relevant for MOLAP servers.