The design of a centralised database amounts to:
1) Designing to "conceptual schema" which
describes the integrated database.
2) Designing the ”physical database" i.e.,
mapping the conceptual schema to storage area and determining appropriate
access methods.
In a distributed
database these two problems become the design of the global schema and the
design of the local physical database at each site, the technique which can be
applied to these problems are the same as in centralised database.
The distribution of the database adds to the above
problems two new ones:
1) Designing the fragmentation, i.e., determining the
global relations are subdivided into horizontal, vertical or mixed fragments.
2) Designing the allocation of fragment, i.e.,
determining how fragments are mapped to physical image.
These two problems fully characterize the design of
data distribution
The distinction between these two problems is
conceptually relevant, since the first one deal with the logical criteria which
motivates the fragmentation of a global relation, while the second one deals
with the "physical" placement of data at the various sites.
