CREATE TYPE() SQL Commands CREATE TYPE()
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 ]
[ , 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).
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. Gener-
ally these functions have to be coded in C or another low-level lan-
guage.
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 transforma-
tion. 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 destina-
tion column, if known (-1 will be passed if not). The input function
must return a value of the data type itself. Usually, an input func-
tion 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 rep-
resentation to the internal representation. If this function is not
supplied, the type cannot participate in binary input. The binary rep-
resentation 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 representa-
tion, 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 tak-
ing 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 argu-
ments are the same as for the text input function. The receive func-
tion 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 func-
tion 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_func-
tion converts from the internal representation to the external binary
representation. If this function is not supplied, the type cannot par-
ticipate 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 func-
tions 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 referenc-
ing 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_out-
put_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_func-
tion 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_func-
tion. 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 dis-
play format is just the stored typmod integer value enclosed in paren-
theses.
The optional analyze_function performs type-specific statistics collec-
tion for columns of the data type. By default, ANALYZE will attempt to
gather statistics using the type's ``equals'' and ``less-than'' opera-
tors, 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 func-
tion 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 internal-
length. Base data types can be fixed-length, in which case internal-
length 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 align-
ment of at least 4, since they necessarily contain an int4 as their
first component.
The storage parameter allows selection of storage strategies for vari-
able-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 ta-
ble 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.)
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 overrid-
den by an explicit DEFAULT clause attached to a particular column.)
To indicate that a type is an array, specify the type of the array ele-
ments 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 spe-
cific 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 cre-
ates 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-collid-
ing 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 inter-
nal form is exactly a sequence of identical fixed-length fields. A sub-
scriptable variable-length type must have the generalized internal rep-
resentation 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), sub-
scripting 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 com-
posite 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 exter-
nal textual form to its internal form.
output_function
The name of a function that converts data from the type's inter-
nal form to its external textual form.
receive_function
The name of a function that converts data from the type's exter-
nal binary form to its internal form.
send_function
The name of a function that converts data from the type's inter-
nal 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 speci-
fied, 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.
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. (The cre-
ator of the type is therefore required to own these functions.) 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 confu-
sion, 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 func-
tion 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 refer-
enced 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 argu-
ments and results also had to be declared as opaque before 7.3. To sup-
port loading of old dump files, CREATE TYPE will accept I/O functions
declared using opaque, but it will issue a notice and change the func-
tion declarations to use the correct types.
EXAMPLES
This example creates a composite type and uses it in a function defini-
tion:
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 defini-
tion:
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 defini-
tion:
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(l)], DROP TYPE [drop_type(l)], ALTER
TYPE [alter_type(l)], CREATE DOMAIN [create_domain(l)]
SQL - Language Statements 2008-09-19 CREATE TYPE()
postgresql 8.3.4 - Generated Fri Oct 3 18:29:57 CDT 2008