37.2. The PostgreSQL Type System
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## 37.2. The PostgreSQL Type System

PostgreSQL data types are divided into base types, composite types, domains, and pseudo-types.

### 37.2.1. Base Types

Base types are those, like `int4`, that are implemented below the level of the SQL language (typically in a low-level language such as C). They generally correspond to what are often known as abstract data types. PostgreSQL can only operate on such types through functions provided by the user and only understands the behavior of such types to the extent that the user describes them. Base types are further subdivided into scalar and array types. For each scalar type, a corresponding array type is automatically created that can hold variable-size arrays of that scalar type.

### 37.2.2. Composite Types

Composite types, or row types, are created whenever the user creates a table. It is also possible to use CREATE TYPE to define a “stand-alone” composite type with no associated table. A composite type is simply a list of types with associated field names. A value of a composite type is a row or record of field values. The user can access the component fields from SQL queries. Refer to Section 8.16 for more information on composite types.

### 37.2.3. Domains

A domain is based on a particular base type and for many purposes is interchangeable with its base type. However, a domain can have constraints that restrict its valid values to a subset of what the underlying base type would allow.

Domains can be created using the SQL command CREATE DOMAIN. Their creation and use is not discussed in this chapter.

### 37.2.4. Pseudo-Types

There are a few “pseudo-types” for special purposes. Pseudo-types cannot appear as columns of tables or attributes of composite types, but they can be used to declare the argument and result types of functions. This provides a mechanism within the type system to identify special classes of functions. Table 8.25 lists the existing pseudo-types.

### 37.2.5. Polymorphic Types

Five pseudo-types of special interest are `anyelement`, `anyarray`, `anynonarray`, `anyenum`, and `anyrange`, which are collectively called polymorphic types. Any function declared using these types is said to be a polymorphic function. A polymorphic function can operate on many different data types, with the specific data type(s) being determined by the data types actually passed to it in a particular call.

Polymorphic arguments and results are tied to each other and are resolved to a specific data type when a query calling a polymorphic function is parsed. Each position (either argument or return value) declared as `anyelement` is allowed to have any specific actual data type, but in any given call they must all be the same actual type. Each position declared as `anyarray` can have any array data type, but similarly they must all be the same type. And similarly, positions declared as `anyrange` must all be the same range type. Furthermore, if there are positions declared `anyarray` and others declared `anyelement`, the actual array type in the `anyarray` positions must be an array whose elements are the same type appearing in the `anyelement` positions. Similarly, if there are positions declared `anyrange` and others declared `anyelement`, the actual range type in the `anyrange` positions must be a range whose subtype is the same type appearing in the `anyelement` positions. `anynonarray` is treated exactly the same as `anyelement`, but adds the additional constraint that the actual type must not be an array type. `anyenum` is treated exactly the same as `anyelement`, but adds the additional constraint that the actual type must be an enum type.

Thus, when more than one argument position is declared with a polymorphic type, the net effect is that only certain combinations of actual argument types are allowed. For example, a function declared as `equal(anyelement, anyelement)` will take any two input values, so long as they are of the same data type.

When the return value of a function is declared as a polymorphic type, there must be at least one argument position that is also polymorphic, and the actual data type supplied as the argument determines the actual result type for that call. For example, if there were not already an array subscripting mechanism, one could define a function that implements subscripting as `subscript(anyarray, integer) returns anyelement`. This declaration constrains the actual first argument to be an array type, and allows the parser to infer the correct result type from the actual first argument's type. Another example is that a function declared as `f(anyarray) returns anyenum` will only accept arrays of enum types.

Note that `anynonarray` and `anyenum` do not represent separate type variables; they are the same type as `anyelement`, just with an additional constraint. For example, declaring a function as `f(anyelement, anyenum)` is equivalent to declaring it as `f(anyenum, anyenum)`: both actual arguments have to be the same enum type.

A variadic function (one taking a variable number of arguments, as in Section 37.4.5) can be polymorphic: this is accomplished by declaring its last parameter as `VARIADIC` `anyarray`. For purposes of argument matching and determining the actual result type, such a function behaves the same as if you had written the appropriate number of `anynonarray` parameters.

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