The signature of an instance or constructor method is the partial definition that you must provide when you create a UDT (see “CREATE TYPE (Distinct Form)” on page 746 and “CREATE TYPE (Structured Form)” on page 758) or add an instance or constructor method to the UDT definition (see “ALTER TYPE” on page 148).
The method signature specified for a UDT definition does not completely define an instance or constructor method; however it does canonically define the following aspects of an instance or constructor method:
The method signature also defines, possibly redundantly, the data type it returns. Note that the RETURNS clause is mandatory for the method signature, but optional within the clauses included within CREATE METHOD. The definitions must be identical if both are specified.
A method signature must redundantly and identically specify the following list of items from its associated method body.
You do not need to specify the INSTANCE keyword if its associated CREATE METHOD statement explicitly defines it as such because if no keyword is specified for a method, it is always assumed to be an instance method by default.
You must always create the method body using the CREATE METHOD statement.