CREATE TYPE

CREATE TYPE

7

SQL - Language Statements

CREATE TYPE

define a new data type

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

\] \[ , MULTIRANGE\_TYPE\_NAME =

multirange\_type\_name

\] ) 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

\] \[ , SUBSCRIPT =

subscript\_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 {#sql-createtype-enum}
----------------

The second form of `CREATE TYPE` creates an enumerated (enum) type, as
described in [???](#datatype-enum). 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`](#sql-altertype).)

Range Types {#sql-createtype-range}
-----------

The third form of `CREATE TYPE` creates a new range type, as described
in [???](#rangetypes).

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
[???](#rangetypes-defining) 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 [???](#rangetypes-defining) for more information.

The optional multirange\_type\_name parameter specifies the name of the
corresponding multirange type. If not specified, this name is chosen
automatically as follows. If the range type name contains the substring
`range`, then the multirange type name is formed by replacement of the
`range` substring with `multirange` in the range type name. Otherwise,
the multirange type name is formed by appending a `_multirange` suffix
to the range type name.

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, analyze\_function, and
subscript\_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`.

The optional subscript\_function allows the data type to be subscripted
in SQL commands. Specifying this function does not cause the type to be
considered a "true" array type; for example, it will not be a candidate
for the result type of `ARRAY[]` constructs. But if subscripting a value
of the type is a natural notation for extracting data from it, then a
subscript\_function can be written to define what that means. The
subscript function must be declared to take a single argument of type
`internal`, and return an `internal` result, which is a pointer to a
struct of methods (functions) that implement subscripting. The detailed
API for subscript functions appears in
`src/include/nodes/subscripting.h`. It may also be useful to read the
array implementation in `src/backend/utils/adt/arraysubs.c`, or the
simpler code in `contrib/hstore/hstore_subs.c`. Additional information
appears in [Array Types](#sql-createtype-array) below.

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 [???](#storage-toast); 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
[???](#storage-toast) and [???](#xtypes-toast). 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 [???](#typeconv). 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 a fixed-length array type, 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`.
For more details, see [Array Types](#sql-createtype-array) 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 {#sql-createtype-array}
-----------

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`. Furthermore, this type is what the system
uses for constructs such as `ARRAY[]` over the user-defined type. 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 main 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. 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.

Specifying the `SUBSCRIPT` option allows a data type to be subscripted,
even though the system does not otherwise regard it as an array type.
The behavior just described for fixed-length arrays is actually
implemented by the `SUBSCRIPT` handler function
`raw_array_subscript_handler`, which is used automatically if you
specify `ELEMENT` for a fixed-length type without also writing
`SUBSCRIPT`.

When specifying a custom `SUBSCRIPT` function, it is not necessary to
specify `ELEMENT` unless the `SUBSCRIPT` handler function needs to
consult typelem to find out what to return. Be aware that specifying
`ELEMENT` causes the system to assume that the new type contains, or is
somehow physically dependent on, the element type; thus for example
changing properties of the element type won\'t be allowed if there are
any columns of the dependent type.

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.

multirange\_type\_name

:   The name of the corresponding multirange type.

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.

subscript\_function

:   The name of a function that defines what subscripting a value of the
    data type does.

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 [???](#catalog-typcategory-table). 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 {#sql-createtype-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.

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
[???](#xtypes).

Compatibility {#sql-createtype-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 {#sql-createtype-see-also}
========
