The federated database
To end users and client applications, data sources appear as a single collective database in the 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 non-relational 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. 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.