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Unit 6: Data Warehousing

viewed as read-only databases. This satisfies the users’ need for a short analysis query response Notes
time and has other important effects. First, it affects data warehouse–specific database management
system (DBMS) technologies, because there is no need for advanced transaction management
techniques required by operational applications. Second, data warehouses operate in read-only
mode, so data warehouse–specific logical design solutions are completely different from those
used for operational databases. For instance, the most obvious feature of data warehouse relational
implementations is that table normalization can be given up to partially denormalize tables
and improve performance.
Other differences between operational databases and data warehouses are connected with query
types. Operational queries execute transactions that generally read/write a small number of
tuples from/to many tables connected by simple relations.

Example: This applies if you search for the data of a customer in order to insert a new
customer order. This kind of query is an OLTP query. On the contrary, the type of query required
in data warehouses is OLAP. It features dynamic, multidimensional analyses that need to scan a
huge amount of records to process a set of numeric data summing up the performance of an
enterprise.

Notes OLTP systems have an essential workload core “frozen” in application programs,
and ad hoc data queries are occasionally run for data maintenance. Conversely, data
warehouse interactivity is an essential property for analysis sessions, so the actual
workload constantly changes as time goes by.
The distinctive features of OLAP queries suggest adoption of a multidimensional representation
for data warehouse data. Basically, data is viewed as points in space, whose dimensions correspond
to many possible analysis dimensions. Each point represents an event that occurs in an enterprise
and is described by a set of measures relevant to decision making processes.

Table 6.1 summarizes the main differences between operational databases and data warehouses.
Table 6.1: Differences between Operational Databases and Data Warehouses

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