PostgreSQL 8.3.0 Documentation
CREATE TYPE

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The optional type_modifier_input_function and type_modifier_output_function are needed if the type supports modifiers, that is optional constraints attached to a type declaration , such as char( 5 ) or numeric( 30,2 ). PostgreSQL allows user-defined types to take one or more simple constants or identifiers as modifiers; however, this information must be capable of being packed into a single non-negative integer value for storage in the system catalogs.