The federated database system catalog
The federated database system catalog contains information about the objects in the federated database and information about objects at the data sources.
The catalog in a federated database is called the global catalog because it contains information about the entire federated system. The query optimizer uses the information in the global catalog and the data source wrapper to plan the best way to process SQL statements. The information stored in the global catalog includes remote and local information, such as column names, column data types, column default values, index information, and statistics information.
Remote catalog information is the information or name used by the data source. Local catalog information is the information or name used by the federated database. For example, suppose a remote table includes a column with the name of EMPNO. The global catalog would store the remote column name as EMPNO. Unless you designate a different name, the local column name will be stored as EMPNO. You can change the local column name to Employee_Number. Users submitting queries which include this column will use Employee_Number in their queries instead of EMPNO. You use the ALTER NICKNAME statement to change the local name of the data source columns.
For relational and nonrelational data sources, the information stored in the global catalog includes both remote and local information.
To see the data source table information that is stored in the global catalog, query the SYSCAT.TABLES, SYSCAT.NICKNAMES, SYSCAT.TABOPTIONS, SYSCAT.INDEXES, SYSCAT.INDEXOPTIONS, SYSCAT.COLUMNS, and SYSCAT.COLOPTIONS catalog views in the federated database.
The global catalog also includes other information about the data sources. For example, the global catalog includes information that the federated server uses to connect to the data source and map the federated user authorizations to the data source user authorizations. The global catalog contains attributes about the data source that you explicitly set, such as server options.