The system catalog is a reflection of the capabilities and features of the DBMS that uses it. Because of the many SQL extensions and additional features offered by popular DBMS products, their system catalogs always contain several tables unique to the DBMS. Here are just a few examples:
- DB2 and Oracle support aliases and synonyms (alternate names for tables). DB2 stores alias information with other table information in the SYSCAT.TABLES system table. Oracle makes synonym information available through its USER_SYNONYMS system view.
- SQL Server supports multiple named databases. It has a system table called SYSDATABASES that identifies the databases managed by a single server.
- Many DBMS brands now support stored procedures, and the catalog contains one or more tables that describes them. Sybase stores information about stored procedures in its SYSPROCEDURES system table.
- Ingres supports tables that are distributed across several disk volumes. Its IIMULTI_LOCATIONS system table keeps track of the locations of multivolume tables.
Source: Liang Y. Daniel (2013), Introduction to programming with SQL, Pearson; 3rd edition.