37.2. The PostgreSQL Type System
PostgreSQL data types can be divided into base types, container types, domains, and pseudo-types.
37.2.1. Base Types
Base types are those, like
, that are
implemented below the level of the
(typically in a low-level language such as C). They generally
correspond to what are often known as abstract data types.
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
The built-in base types are described in
Enumerated (enum) types can be considered as a subcategory of base types. The main difference is that they can be created using just SQL commands, without any low-level programming. Refer to Section 8.7 for more information.
37.2.2. Container Types
PostgreSQL has three kinds of " container " types, which are types that contain multiple values of other types. These are arrays, composites, and ranges.
Arrays can hold multiple values that are all of the same type. An array type is automatically created for each base type, composite type, range type, and domain type. But there are no arrays of arrays. So far as the type system is concerned, multi-dimensional arrays are the same as one-dimensional arrays. Refer to Section 8.15 for more information.
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. Refer to Section 8.16 for more information.
A range type can hold two values of the same type, which are the lower and upper bounds of the range. Range types are user-created, although a few built-in ones exist. Refer to Section 8.17 for more information.
A domain is based on a particular underlying type and for many purposes is interchangeable with its underlying type. However, a domain can have constraints that restrict its valid values to a subset of what the underlying type would allow. Domains are created using the SQL command CREATE DOMAIN . Refer to Section 8.18 for more information.
There are a few " pseudo-types " for special purposes. Pseudo-types cannot appear as columns of tables or components of container 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 8.27 lists the existing pseudo-types.
37.2.5. Polymorphic Types
Some pseudo-types of special interest are the polymorphic types , which are used to declare polymorphic functions . This powerful feature allows a single function definition to 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. The polymorphic types are shown in Table 37.1 . Some examples of their use appear in Section 37.5.10 .
Table 37.1. Polymorphic Types
||Simple||Indicates that a function accepts any data type|
||Simple||Indicates that a function accepts any array data type|
||Simple||Indicates that a function accepts any non-array data type|
||Simple||Indicates that a function accepts any enum data type (see Section 8.7 )|
||Simple||Indicates that a function accepts any range data type (see Section 8.17 )|
||Common||Indicates that a function accepts any data type, with automatic promotion of multiple arguments to a common data type|
||Common||Indicates that a function accepts any array data type, with automatic promotion of multiple arguments to a common data type|
||Common||Indicates that a function accepts any non-array data type, with automatic promotion of multiple arguments to a common data type|
||Common||Indicates that a function accepts any range data type, with automatic promotion of multiple arguments to a common data type|
Polymorphic arguments and results are tied to each other and are resolved to specific data types when a query calling a polymorphic function is parsed. When there is more than one polymorphic argument, the actual data types of the input values must match up as described below. If the function's result type is polymorphic, or it has output parameters of polymorphic types, the types of those results are deduced from the actual types of the polymorphic inputs as described below.
For the " simple " family of polymorphic types, the matching and deduction rules work like this:
Each position (either argument or return value) declared as
is allowed to have any specific actual
data type, but in any given call they must all be the
actual type. Each
position declared as
can have any array data type,
but similarly they must all be the same type. And similarly,
positions declared as
must all be the same range
type. Furthermore, if there are
and others declared
, the actual array type in the
positions must be an array whose elements are
the same type appearing in the
Similarly, if there are positions declared
and others declared
the actual range type in the
positions must be a
range whose subtype is the same type appearing in
positions and the same as the element type
is treated exactly the same as
but adds the additional constraint that the actual type must not be
an array type.
is treated exactly the same as
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
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(s) supplied for the polymorphic arguments
determine 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
. 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.
In most cases, the parser can infer the actual data type for a
polymorphic result type from arguments that are of a different
polymorphic type in the same family; for example
can be deduced from
or vice versa.
An exception is that a
polymorphic result of type
requires an argument
; it cannot be deduced
is because there could be multiple range types with the same subtype.
do not represent
separate type variables; they are the same type as
, just with an additional constraint. For
example, declaring a function as
is equivalent to declaring it as
both actual arguments have to be the same enum type.
family of polymorphic types, the
matching and deduction rules work approximately the same as for
family, with one major difference: the
actual types of the arguments need not be identical, so long as they
can be implicitly cast to a single common type. The common type is
selected following the same rules as for
related constructs (see
Selection of the common type considers the actual types
inputs, the array element types of
inputs, and the range subtypes of
is present then the
common type is required to be a non-array type. Once a common type is
identified, arguments in
positions are automatically
cast to that type, and arguments in
positions are automatically cast to the array type for that type.
Since there is no way to select a range type knowing only its subtype,
requires that all arguments
declared with that type have the same actual range type, and that that
type's subtype agree with the selected common type, so that no casting
of the range values is required. As with
as a function result type requires
that there be an
Notice that there is no
type. Such a
type would not be very useful, since there normally are not any
implicit casts to enum types, meaning that there would be no way to
resolve a common type for dissimilar enum inputs.
The " simple " and " common " polymorphic families represent two independent sets of type variables. Consider for example
CREATE FUNCTION myfunc(a anyelement, b anyelement, c anycompatible, d anycompatible) RETURNS anycompatible AS ...
In an actual call of this function, the first two inputs must have exactly the same type. The last two inputs must be promotable to a common type, but this type need not have anything to do with the type of the first two inputs. The result will have the common type of the last two inputs.
A variadic function (one taking a variable number of arguments, as in
) can be
polymorphic: this is accomplished by declaring its last parameter as
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