To end users and client applications, data sources appear
as a single collective database in the DB2® database
system. Users and applications interface with the federated database that
is managed by the federated server.
The federated database contains a system catalog that
stores information about data. The federated database system catalog
contains entries that identify data sources and their characteristics.
The federated server consults the information stored in the federated
database system catalog and the data source wrapper to determine the
best plan for processing SQL statements.
The federated system
processes SQL statements as if the data from the data sources were
ordinary relational tables or views within the federated database.
As a result:
- The federated system can correlate relational data with data in
nonrelational formats. This is true even when the data sources use
different SQL dialects, or do not support SQL at all.
- The characteristics of the federated database take precedence
when there are differences between the characteristics of the federated
database and the characteristics of the data sources. Query results
conform to DB2 semantics, even if data from other non-DB2
data sources is used to compute the query result.
Examples:
- The code page that the federated server uses is different than
the code page used that the data source uses. In this case, character
data from the data source is converted based on the code page used
by the federated database, when that data is returned to a federated
user.
- The collating sequence that the federated server uses is different
than the collating sequence that the data source uses. In this case,
any sort operations on character data are performed at the federated
server instead of at the data source.