NAME
CREATE TYPE - define a new data type
SYNOPSIS
CREATE TYPE name AS
( attribute_name data_type [, ... ] )
CREATE TYPE name AS ENUM
( ’label’ [, ... ] )
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 ]
)
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.)
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. This 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 as the argument
or return type of a function.
ENUMERATED TYPES
The second form of CREATE TYPE creates an enumerated (enum) type, as
described in in the documentation. Enum types take a list of one or
more quoted labels, each of which must be less than NAMEDATALEN bytes
long (64 in a standard PostgreSQL build).
BASE TYPES
The third 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 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.
The optional flag PASSEDBYVALUE indicates that values of this data type
are passed by value, rather than by reference. You cannot pass by value
types whose internal representation is larger than the size of the
Datum type (4 bytes on most machines, 8 bytes on a few).
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.)
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 in the documentation. 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.
ARRAY TYPES
Whenever a user-defined type is created, PostgreSQL automatically
creates an associated array type, whose name consists of the base
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 it allows to be accessed as 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.
label A string literal representing the textual label associated with
one value of an enum 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.
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 in the documentation. 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.
NOTES
Because there are no restrictions on use of a data type once it’s been
created, creating a base 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
less 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 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 pseudotype 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 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 in
the documentation.
COMPATIBILITY
This CREATE TYPE command is a PostgreSQL extension. There is a CREATE
TYPE statement in the SQL standard that is rather different in detail.
SEE ALSO
CREATE FUNCTION [create_function(7)], DROP TYPE [drop_type(7)], ALTER
TYPE [alter_type(7)], CREATE DOMAIN [create_domain(7)]