Database Reference
In-Depth Information
Global
External
Schema
Local
External
Schema
Local
Conceptual
Schema
Local
External
Schema
Local
External
Schema
Local
data
DB
Local
Internal
Schema
DBMS
London
Local
External
Schema
Global
External
Schema
DDBMS
Global
Conceptual
Schema
Local
Conceptual
Schema
Local
External
Schema
Local
External
Schema
Global
External
Schema
DBMS
Local
data
DB
Local
Conceptual
Schema
Local
Internal
Schema
Local
Internal
Schema
Tokyo
Local
data
DB
DBMS
New York
Figure 18-6
Heterogeneous system: schema levels.
•
An allocation schema
•
For each site, a set of local mapping schema, local conceptual schema, local
internal schema
The global conceptual schema logically describes the entire database. It is a union
of all the local conceptual schemas. Global external schemas represent user views
containing data from different sites. The local mapping schema maps portions of
data allocated to different sites to the local external schemas.
The fragmentation schema indicates how data are divided up through parti-
tioning or replication. The allocation schema indicates where fragmented parts
are to be located. We will cover fragmentation and allocation further in a later
subsection.
Heterogeneous system: schema levels
Figure 18-6 shows a configuration with
the following schemas:
•
A collection of global external schemas
•
A single global conceptual schema
•
For each site, a set of local external schemas
•
For each site, a local conceptual schema
•
For each site, a local internal schema
The global conceptual schema in this case is not necessarily a complete union
of all the local conceptual schemas. It is a subset of the collection of all the local
schemas. It consists of only those parts of the local data that each site agrees to
share.
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