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Figure 13. Schema versioning graph for the
university case
V1.0 and V2.1. Table 3 shows the way in which
data can be shared between versions of the data
warehouse. This is in accordance with the method
given in section 5.2.
For retrieval from the MVDW, let us consider
a query that retrieves the number of students per
year and program. By using the query exten-
sion approach (given in section 7.1), the SQL
becomes:
Select
P r o g r a m N a m e ,
y e a r ,
sum(passedstudents)
From
Program, Universtyfact, Time
Where
program
joinwith
university fact
And
universityfact
joinwith
time
Group by
programname, year
Version begin date
'T1'
Merge into
'V0.0';
In contrast, the same query using the transpar-
ent querying method (given in section 7.2) can
be written as:
Select
There is no schema change in V1.0, therefore
schema does not need to be shared between V1.0
and V0.0. In contrast, in V2.1 schema change
takes place by adding a level 'LP' to one of the
dimensions. Therefore, schema sharing takes place
between V1.0 and V2.1.
In order to keep the size of the data warehouse
small, data stored by the version V0.0 is shared by
P r o g r a m N a m e ,
y e a r ,
sum(passedstudents)
Figure 14. Modeling schema sharing for the university case
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