The PostgreSQL type system can be broken down in several ways. Types are divided into base types and composite types. Base types are those, like int4, that are implemented in a language such as C. They generally correspond to what are often known as abstract data types; PostgreSQL can only operate on such types through methods provided by the user and only understands the behavior of such types to the extent that the user describes them. Composite types are created whenever the user creates a table.
PostgreSQL stores these types in only one way (within the file that stores all rows of a table) but the user can "look inside" at the attributes of these types from the query language and optimize their retrieval by (for example) defining indexes on the attributes. PostgreSQL base types are further divided into built-in types and user-defined types. Built-in types (like int4) are those that are compiled into the system. User-defined types are those created by the user in the manner to be described later.