Provided by: postgresql-client-12_12.20-0ubuntu0.20.04.1_amd64 
NAME
CREATE_TYPE - define a new data type
SYNOPSIS
CREATE TYPE name AS
( [ attribute_name data_type [ COLLATE collation ] [, ... ] ] )
CREATE TYPE name AS ENUM
( [ 'label' [, ... ] ] )
CREATE TYPE name AS RANGE (
SUBTYPE = subtype
[ , SUBTYPE_OPCLASS = subtype_operator_class ]
[ , COLLATION = collation ]
[ , CANONICAL = canonical_function ]
[ , SUBTYPE_DIFF = subtype_diff_function ]
)
CREATE TYPE name (
INPUT = input_function,
OUTPUT = output_function
[ , RECEIVE = receive_function ]
[ , SEND = send_function ]
[ , TYPMOD_IN = type_modifier_input_function ]
[ , TYPMOD_OUT = type_modifier_output_function ]
[ , ANALYZE = analyze_function ]
[ , INTERNALLENGTH = { internallength | VARIABLE } ]
[ , PASSEDBYVALUE ]
[ , ALIGNMENT = alignment ]
[ , STORAGE = storage ]
[ , LIKE = like_type ]
[ , CATEGORY = category ]
[ , PREFERRED = preferred ]
[ , DEFAULT = default ]
[ , ELEMENT = element ]
[ , DELIMITER = delimiter ]
[ , COLLATABLE = collatable ]
)
CREATE TYPE name
DESCRIPTION
CREATE TYPE registers a new data type for use in the current database. The user who
defines a type becomes its owner.
If a schema name is given then the type is created in the specified schema. Otherwise it
is created in the current schema. The type name must be distinct from the name of any
existing type or domain in the same schema. (Because tables have associated data types,
the type name must also be distinct from the name of any existing table in the same
schema.)
There are five forms of CREATE TYPE, as shown in the syntax synopsis above. They
respectively create a composite type, an enum type, a range type, a base type, or a shell
type. The first four of these are discussed in turn below. A shell type is simply a
placeholder for a type to be defined later; it is created by issuing CREATE TYPE with no
parameters except for the type name. Shell types are needed as forward references when
creating range types and base types, as discussed in those sections.
Composite Types
The first form of CREATE TYPE creates a composite type. The composite type is specified by
a list of attribute names and data types. An attribute's collation can be specified too,
if its data type is collatable. A composite type is essentially the same as the row type
of a table, but using CREATE TYPE avoids the need to create an actual table when all that
is wanted is to define a type. A stand-alone composite type is useful, for example, as the
argument or return type of a function.
To be able to create a composite type, you must have USAGE privilege on all attribute
types.
Enumerated Types
The second form of CREATE TYPE creates an enumerated (enum) type, as described in
Section 8.7. Enum types take a list of quoted labels, each of which must be less than
NAMEDATALEN bytes long (64 bytes in a standard PostgreSQL build). (It is possible to
create an enumerated type with zero labels, but such a type cannot be used to hold values
before at least one label is added using ALTER TYPE (ALTER_TYPE(7)).)
Range Types
The third form of CREATE TYPE creates a new range type, as described in Section 8.17.
The range type's subtype can be any type with an associated b-tree operator class (to
determine the ordering of values for the range type). Normally the subtype's default
b-tree operator class is used to determine ordering; to use a non-default operator class,
specify its name with subtype_opclass. If the subtype is collatable, and you want to use a
non-default collation in the range's ordering, specify the desired collation with the
collation option.
The optional canonical function must take one argument of the range type being defined,
and return a value of the same type. This is used to convert range values to a canonical
form, when applicable. See Section 8.17.8 for more information. Creating a canonical
function is a bit tricky, since it must be defined before the range type can be declared.
To do this, you must first create a shell type, which is a placeholder type that has no
properties except a name and an owner. This is done by issuing the command CREATE TYPE
name, with no additional parameters. Then the function can be declared using the shell
type as argument and result, and finally the range type can be declared using the same
name. This automatically replaces the shell type entry with a valid range type.
The optional subtype_diff function must take two values of the subtype type as argument,
and return a double precision value representing the difference between the two given
values. While this is optional, providing it allows much greater efficiency of GiST
indexes on columns of the range type. See Section 8.17.8 for more information.
Base Types
The fourth form of CREATE TYPE creates a new base type (scalar type). To create a new base
type, you must be a superuser. (This restriction is made because an erroneous type
definition could confuse or even crash the server.)
The parameters can appear in any order, not only that illustrated above, and most are
optional. You must register two or more functions (using CREATE FUNCTION) before defining
the type. The support functions input_function and output_function are required, while the
functions receive_function, send_function, type_modifier_input_function,
type_modifier_output_function and analyze_function are optional. Generally these functions
have to be coded in C or another low-level language.
The input_function converts the type's external textual representation to the internal
representation used by the operators and functions defined for the type. output_function
performs the reverse transformation. The input function can be declared as taking one
argument of type cstring, or as taking three arguments of types cstring, oid, integer. The
first argument is the input text as a C string, the second argument is the type's own OID
(except for array types, which instead receive their element type's OID), and the third is
the typmod of the destination column, if known (-1 will be passed if not). The input
function must return a value of the data type itself. Usually, an input function should be
declared STRICT; if it is not, it will be called with a NULL first parameter when reading
a NULL input value. The function must still return NULL in this case, unless it raises an
error. (This case is mainly meant to support domain input functions, which might need to
reject NULL inputs.) The output function must be declared as taking one argument of the
new data type. The output function must return type cstring. Output functions are not
invoked for NULL values.
The optional receive_function converts the type's external binary representation to the
internal representation. If this function is not supplied, the type cannot participate in
binary input. The binary representation should be chosen to be cheap to convert to
internal form, while being reasonably portable. (For example, the standard integer data
types use network byte order as the external binary representation, while the internal
representation is in the machine's native byte order.) The receive function should perform
adequate checking to ensure that the value is valid. The receive function can be declared
as taking one argument of type internal, or as taking three arguments of types internal,
oid, integer. The first argument is a pointer to a StringInfo buffer holding the received
byte string; the optional arguments are the same as for the text input function. The
receive function must return a value of the data type itself. Usually, a receive function
should be declared STRICT; if it is not, it will be called with a NULL first parameter
when reading a NULL input value. The function must still return NULL in this case, unless
it raises an error. (This case is mainly meant to support domain receive functions, which
might need to reject NULL inputs.) Similarly, the optional send_function converts from the
internal representation to the external binary representation. If this function is not
supplied, the type cannot participate in binary output. The send function must be declared
as taking one argument of the new data type. The send function must return type bytea.
Send functions are not invoked for NULL values.
You should at this point be wondering how the input and output functions can be declared
to have results or arguments of the new type, when they have to be created before the new
type can be created. The answer is that the type should first be defined as a shell type,
which is a placeholder type that has no properties except a name and an owner. This is
done by issuing the command CREATE TYPE name, with no additional parameters. Then the C
I/O functions can be defined referencing the shell type. Finally, CREATE TYPE with a full
definition replaces the shell entry with a complete, valid type definition, after which
the new type can be used normally.
The optional type_modifier_input_function and type_modifier_output_function are needed if
the type supports modifiers, that is optional constraints attached to a type declaration,
such as char(5) or numeric(30,2). PostgreSQL allows user-defined types to take one or
more simple constants or identifiers as modifiers. However, this information must be
capable of being packed into a single non-negative integer value for storage in the system
catalogs. The type_modifier_input_function is passed the declared modifier(s) in the form
of a cstring array. It must check the values for validity (throwing an error if they are
wrong), and if they are correct, return a single non-negative integer value that will be
stored as the column “typmod”. Type modifiers will be rejected if the type does not have a
type_modifier_input_function. The type_modifier_output_function converts the internal
integer typmod value back to the correct form for user display. It must return a cstring
value that is the exact string to append to the type name; for example numeric's function
might return (30,2). It is allowed to omit the type_modifier_output_function, in which
case the default display format is just the stored typmod integer value enclosed in
parentheses.
The optional analyze_function performs type-specific statistics collection for columns of
the data type. By default, ANALYZE will attempt to gather statistics using the type's
“equals” and “less-than” operators, if there is a default b-tree operator class for the
type. For non-scalar types this behavior is likely to be unsuitable, so it can be
overridden by specifying a custom analysis function. The analysis function must be
declared to take a single argument of type internal, and return a boolean result. The
detailed API for analysis functions appears in src/include/commands/vacuum.h.
While the details of the new type's internal representation are only known to the I/O
functions and other functions you create to work with the type, there are several
properties of the internal representation that must be declared to PostgreSQL. Foremost of
these is internallength. Base data types can be fixed-length, in which case internallength
is a positive integer, or variable-length, indicated by setting internallength to
VARIABLE. (Internally, this is represented by setting typlen to -1.) The internal
representation of all variable-length types must start with a 4-byte integer giving the
total length of this value of the type. (Note that the length field is often encoded, as
described in Section 69.2; it's unwise to access it directly.)
The optional flag PASSEDBYVALUE indicates that values of this data type are passed by
value, rather than by reference. Types passed by value must be fixed-length, and their
internal representation cannot be larger than the size of the Datum type (4 bytes on some
machines, 8 bytes on others).
The alignment parameter specifies the storage alignment required for the data type. The
allowed values equate to alignment on 1, 2, 4, or 8 byte boundaries. Note that
variable-length types must have an alignment of at least 4, since they necessarily contain
an int4 as their first component.
The storage parameter allows selection of storage strategies for variable-length data
types. (Only plain is allowed for fixed-length types.) plain specifies that data of the
type will always be stored in-line and not compressed. extended specifies that the system
will first try to compress a long data value, and will move the value out of the main
table row if it's still too long. external allows the value to be moved out of the main
table, but the system will not try to compress it. main allows compression, but
discourages moving the value out of the main table. (Data items with this storage strategy
might still be moved out of the main table if there is no other way to make a row fit, but
they will be kept in the main table preferentially over extended and external items.)
All storage values other than plain imply that the functions of the data type can handle
values that have been toasted, as described in Section 69.2 and Section 37.13.1. The
specific other value given merely determines the default TOAST storage strategy for
columns of a toastable data type; users can pick other strategies for individual columns
using ALTER TABLE SET STORAGE.
The like_type parameter provides an alternative method for specifying the basic
representation properties of a data type: copy them from some existing type. The values of
internallength, passedbyvalue, alignment, and storage are copied from the named type. (It
is possible, though usually undesirable, to override some of these values by specifying
them along with the LIKE clause.) Specifying representation this way is especially useful
when the low-level implementation of the new type “piggybacks” on an existing type in some
fashion.
The category and preferred parameters can be used to help control which implicit cast will
be applied in ambiguous situations. Each data type belongs to a category named by a single
ASCII character, and each type is either “preferred” or not within its category. The
parser will prefer casting to preferred types (but only from other types within the same
category) when this rule is helpful in resolving overloaded functions or operators. For
more details see Chapter 10. For types that have no implicit casts to or from any other
types, it is sufficient to leave these settings at the defaults. However, for a group of
related types that have implicit casts, it is often helpful to mark them all as belonging
to a category and select one or two of the “most general” types as being preferred within
the category. The category parameter is especially useful when adding a user-defined type
to an existing built-in category, such as the numeric or string types. However, it is also
possible to create new entirely-user-defined type categories. Select any ASCII character
other than an upper-case letter to name such a category.
A default value can be specified, in case a user wants columns of the data type to default
to something other than the null value. Specify the default with the DEFAULT key word.
(Such a default can be overridden by an explicit DEFAULT clause attached to a particular
column.)
To indicate that a type is an array, specify the type of the array elements using the
ELEMENT key word. For example, to define an array of 4-byte integers (int4), specify
ELEMENT = int4. More details about array types appear below.
To indicate the delimiter to be used between values in the external representation of
arrays of this type, delimiter can be set to a specific character. The default delimiter
is the comma (,). Note that the delimiter is associated with the array element type, not
the array type itself.
If the optional Boolean parameter collatable is true, column definitions and expressions
of the type may carry collation information through use of the COLLATE clause. It is up to
the implementations of the functions operating on the type to actually make use of the
collation information; this does not happen automatically merely by marking the type
collatable.
Array Types
Whenever a user-defined type is created, PostgreSQL automatically creates an associated
array type, whose name consists of the element type's name prepended with an underscore,
and truncated if necessary to keep it less than NAMEDATALEN bytes long. (If the name so
generated collides with an existing type name, the process is repeated until a
non-colliding name is found.) This implicitly-created array type is variable length and
uses the built-in input and output functions array_in and array_out. The array type tracks
any changes in its element type's owner or schema, and is dropped if the element type is.
You might reasonably ask why there is an ELEMENT option, if the system makes the correct
array type automatically. The only case where it's useful to use ELEMENT is when you are
making a fixed-length type that happens to be internally an array of a number of identical
things, and you want to allow these things to be accessed directly by subscripting, in
addition to whatever operations you plan to provide for the type as a whole. For example,
type point is represented as just two floating-point numbers, which can be accessed using
point[0] and point[1]. Note that this facility only works for fixed-length types whose
internal form is exactly a sequence of identical fixed-length fields. A subscriptable
variable-length type must have the generalized internal representation used by array_in
and array_out. For historical reasons (i.e., this is clearly wrong but it's far too late
to change it), subscripting of fixed-length array types starts from zero, rather than from
one as for variable-length arrays.
PARAMETERS
name
The name (optionally schema-qualified) of a type to be created.
attribute_name
The name of an attribute (column) for the composite type.
data_type
The name of an existing data type to become a column of the composite type.
collation
The name of an existing collation to be associated with a column of a composite type,
or with a range type.
label
A string literal representing the textual label associated with one value of an enum
type.
subtype
The name of the element type that the range type will represent ranges of.
subtype_operator_class
The name of a b-tree operator class for the subtype.
canonical_function
The name of the canonicalization function for the range type.
subtype_diff_function
The name of a difference function for the subtype.
input_function
The name of a function that converts data from the type's external textual form to its
internal form.
output_function
The name of a function that converts data from the type's internal form to its
external textual form.
receive_function
The name of a function that converts data from the type's external binary form to its
internal form.
send_function
The name of a function that converts data from the type's internal form to its
external binary form.
type_modifier_input_function
The name of a function that converts an array of modifier(s) for the type into
internal form.
type_modifier_output_function
The name of a function that converts the internal form of the type's modifier(s) to
external textual form.
analyze_function
The name of a function that performs statistical analysis for the data type.
internallength
A numeric constant that specifies the length in bytes of the new type's internal
representation. The default assumption is that it is variable-length.
alignment
The storage alignment requirement of the data type. If specified, it must be char,
int2, int4, or double; the default is int4.
storage
The storage strategy for the data type. If specified, must be plain, external,
extended, or main; the default is plain.
like_type
The name of an existing data type that the new type will have the same representation
as. The values of internallength, passedbyvalue, alignment, and storage are copied
from that type, unless overridden by explicit specification elsewhere in this CREATE
TYPE command.
category
The category code (a single ASCII character) for this type. The default is 'U' for
“user-defined type”. Other standard category codes can be found in Table 51.64. You
may also choose other ASCII characters in order to create custom categories.
preferred
True if this type is a preferred type within its type category, else false. The
default is false. Be very careful about creating a new preferred type within an
existing type category, as this could cause surprising changes in behavior.
default
The default value for the data type. If this is omitted, the default is null.
element
The type being created is an array; this specifies the type of the array elements.
delimiter
The delimiter character to be used between values in arrays made of this type.
collatable
True if this type's operations can use collation information. The default is false.
NOTES
Because there are no restrictions on use of a data type once it's been created, creating a
base type or range type is tantamount to granting public execute permission on the
functions mentioned in the type definition. This is usually not an issue for the sorts of
functions that are useful in a type definition. But you might want to think twice before
designing a type in a way that would require “secret” information to be used while
converting it to or from external form.
Before PostgreSQL version 8.3, the name of a generated array type was always exactly the
element type's name with one underscore character (_) prepended. (Type names were
therefore restricted in length to one fewer character than other names.) While this is
still usually the case, the array type name may vary from this in case of maximum-length
names or collisions with user type names that begin with underscore. Writing code that
depends on this convention is therefore deprecated. Instead, use pg_type.typarray to
locate the array type associated with a given type.
It may be advisable to avoid using type and table names that begin with underscore. While
the server will change generated array type names to avoid collisions with user-given
names, there is still risk of confusion, particularly with old client software that may
assume that type names beginning with underscores always represent arrays.
Before PostgreSQL version 8.2, the shell-type creation syntax CREATE TYPE name did not
exist. The way to create a new base type was to create its input function first. In this
approach, PostgreSQL will first see the name of the new data type as the return type of
the input function. The shell type is implicitly created in this situation, and then it
can be referenced in the definitions of the remaining I/O functions. This approach still
works, but is deprecated and might be disallowed in some future release. Also, to avoid
accidentally cluttering the catalogs with shell types as a result of simple typos in
function definitions, a shell type will only be made this way when the input function is
written in C.
In PostgreSQL versions before 7.3, it was customary to avoid creating a shell type at all,
by replacing the functions' forward references to the type name with the placeholder
pseudo-type opaque. The cstring arguments and results also had to be declared as opaque
before 7.3. To support loading of old dump files, CREATE TYPE will accept I/O functions
declared using opaque, but it will issue a notice and change the function declarations to
use the correct types.
EXAMPLES
This example creates a composite type and uses it in a function definition:
CREATE TYPE compfoo AS (f1 int, f2 text);
CREATE FUNCTION getfoo() RETURNS SETOF compfoo AS $$
SELECT fooid, fooname FROM foo
$$ LANGUAGE SQL;
This example creates an enumerated type and uses it in a table definition:
CREATE TYPE bug_status AS ENUM ('new', 'open', 'closed');
CREATE TABLE bug (
id serial,
description text,
status bug_status
);
This example creates a range type:
CREATE TYPE float8_range AS RANGE (subtype = float8, subtype_diff = float8mi);
This example creates the base data type box and then uses the type in a table definition:
CREATE TYPE box;
CREATE FUNCTION my_box_in_function(cstring) RETURNS box AS ... ;
CREATE FUNCTION my_box_out_function(box) RETURNS cstring AS ... ;
CREATE TYPE box (
INTERNALLENGTH = 16,
INPUT = my_box_in_function,
OUTPUT = my_box_out_function
);
CREATE TABLE myboxes (
id integer,
description box
);
If the internal structure of box were an array of four float4 elements, we might instead
use:
CREATE TYPE box (
INTERNALLENGTH = 16,
INPUT = my_box_in_function,
OUTPUT = my_box_out_function,
ELEMENT = float4
);
which would allow a box value's component numbers to be accessed by subscripting.
Otherwise the type behaves the same as before.
This example creates a large object type and uses it in a table definition:
CREATE TYPE bigobj (
INPUT = lo_filein, OUTPUT = lo_fileout,
INTERNALLENGTH = VARIABLE
);
CREATE TABLE big_objs (
id integer,
obj bigobj
);
More examples, including suitable input and output functions, are in Section 37.13.
COMPATIBILITY
The first form of the CREATE TYPE command, which creates a composite type, conforms to the
SQL standard. The other forms are PostgreSQL extensions. The CREATE TYPE statement in the
SQL standard also defines other forms that are not implemented in PostgreSQL.
The ability to create a composite type with zero attributes is a PostgreSQL-specific
deviation from the standard (analogous to the same case in CREATE TABLE).
SEE ALSO
ALTER TYPE (ALTER_TYPE(7)), CREATE DOMAIN (CREATE_DOMAIN(7)), CREATE FUNCTION
(CREATE_FUNCTION(7)), DROP TYPE (DROP_TYPE(7))