3. Understanding DBMS Server Requirements for User-Defined Data Types : IDs for Data Types and Functions
 
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IDs for Data Types and Functions
You must assign an identification number to all data types, functions, and function instances when they are created. The ID numbers for each type of object (data types, functions, and function instances) occupy a number space that is unique for that object type. An object's ID must be unique within the same class of objects, but does not have to be unique across object classes. For example, a new data type cannot have an ID that is the same as another data type, but it can have an ID number that is the same as a new function's ID number.
Note:  For upward compatibility, make identification numbers for all objects unique across machines.
Identification numbers are stored in system catalogs in databases. You cannot change an ID number without first removing all references to both the ID and the object it represents from the catalogs. All tables, views, database procedures, grants, permits, integrities, and rules that refer to the object must also be destroyed.
Following are the ranges of values that are reserved for IDs and the corresponding data types:
SQL data types: 1 to 8191
User-defined data types: 16384 to 32767