On Querying Data and Metadata in Multiversion Data Warehouse - Data Warehousing Design and Advanced Engineering Applications - page 209

The important functionality of a DW system
that supports evolution is a query language capable
of: (1) querying multiple DW states (versions) that
differ with respect to their schemas and present-
ing query results to a user in a meaningful way
and (2) querying metadata on the history of DW
changes. Designing such a query language is chal-
lenging since DW versions may differ with respect
to their schemas (e.g., one schema version may
store attribute amount , whereas another may not
or may store a corresponding attribute but with
a different name).
Few languages for querying evolving data
warehouses have been proposed so far in the
research literature, e.g., (Mendelzon & Vaisman,
2000; Vaisman & Mendelzon, 2001), (Eder &
Koncilia, 2001; Eder et al., 2002), and (Rizzi &
Golfarelli, 2007). The languages, however, either
are not capable of querying DW versions that dif-
fer with respect to their schemas or do not allow
to query metadata.
A more detail analysis of related approaches
to managing evolution in databases and data
warehouses as well as approaches to querying
evolving databases and data warehouses can be
found in (Wrembel, 2009).

• Queries retrieving the history of the evo-

lution of an indicated DW object (a level
table, fact table, attribute, dimension, di-
mension instance).

In this chapter we discuss the functionality
of MVDWQL queries, their syntax, and how the
queries are processed. We discuss how to handle
queries that address DW versions that differ with
respect to their schemas. We outline our prototype
MVDWQL software. Finally, we summarize the
chapter.

MULTIVERSION DATA WAREHOUSE

DW Versions

The Multiversion Data Warehouse is composed
of the sequence of DW versions. A DW version
is composed of a DW schema version and a
DW instance version. The DW schema version
describes the structure of a DW within a given
time period, whereas the DW instance version
represents the set of data described by its schema
version.
We distinguish two types of DW versions,
namely real and alternative ones (Bębel et al.,
2004). Real versions are created in order to
reflect changes in a real business environment.
Real versions are linearly ordered by the time
they are valid within. Alternative versions are
created mainly for simulation purposes, as part
of the 'what-if' analysis. Such versions represent
virtual business scenarios.
Versions of the MVDW form a DW version
derivation tree . Each node of this three represents
a DW version (real or alternative), whereas edges
represent derived-from relationships. The root of
the DW version derivation tree is the first real
version. Figure 1 schematically shows an example
DW version derivation tree where R i represents
a real version and A i represents an alternative
version.

Chapter Contribution

This chapter contributes the so-called Multiversion
Data Warehouse Query Language ( MVDWQL )
developed for querying the MVDW . The language
proposed in this chapter is the extension of our previ-
ous works (Morzy & Wrembel, 2004; Wrembel &
Bębel, 2007; Wrembel & Morzy, 2006) with respect
to the functionality that allows to explicitly query
metadata on the MVDW . To this end, two types of
queries on metadata are supported, namely:

• Queries searching for DW versions that in-

clude an indicated DW object (a level table,
fact table, attribute, dimension, dimension
instance) or its equivalent existing in other
DW versions;