|PostgreSQL Programmer's Guide|
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The Postgres 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"; Postgres 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 class. EMP is an example of a composite type.
Postgres stores these types in only one way (within the file that stores all instances of the class) but the user can "look inside" at the attributes of these types from the query language and optimize their retrieval by (for example) defining indices on the attributes. Postgres 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 below.
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