PostgreSQL, Second Edition

Defining a Simple Data Type in PostgreSQL

Now that you understand the difference between internal and external forms, it should be obvious that PostgreSQL needs to convert values between these forms. When you define a new data type, you tell PostgreSQL how to convert a value from external form to internal form and from internal form to external form.

Let's create a simple type that mimics the built-in TEXT data type. Data type descriptions are stored in the pg_type system table. We are interested in three of the columns:

movies=# SELECT typinput, typoutput, typlen
movies-#   FROM pg_type
movies-#   WHERE typname = 'text';
 typinput | typoutput | typlen
----------+-----------+--------
 textin   | textout   |     -1

The typinput column tells you the name of ...