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CREATE
NAME
CREATE TYPE - define a new data type
SYNOPSIS
CREATE TYPE name AS
( attribute_name data_type [, ... ] )
CREATE TYPE name (
INPUT = input_function,
OUTPUT = output_function
[ , RECEIVE = receive_function ]
[ , SEND = send_function ]
[ , INTERNALLENGTH = { internallength | VARIABLE } ]
[ , PASSEDBYVALUE ]
[ , ALIGNMENT = alignment ]
[ , STORAGE = storage ]
[ , DEFAULT = default ]
[ , ELEMENT = element ]
[ , DELIMITER = delimiter ]
)
DESCRIPTION
CREATE TYPE registers a new data type for use in the current data base. 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 return type
of a function.
BASE TYPES
The second form of CREATE TYPE creates a new base type (scalar type). The
parameters may 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 and
send_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 may 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 element type in
case this is an array type, and the third is the typmod of the destination
column, if known. The input function should return a value of the data
type itself. The output function may be declared as taking one argument of
the new data type, or as taking two arguments of which the second is type
oid. The second argument is again the array element type for array types.
The output function should return type cstring.
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 may be declared as taking one argument of
type internal, or two arguments of types internal and oid. It must return a
value of the data type itself. (The first argument is a pointer to a
StringInfo buffer holding the received byte string; the optional second
argument is the element type in case this is an array type.) 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 may be declared as
taking one argument of the new data type, or as taking two arguments of
which the second is type oid. The second argument is again the array
element type for array types. The send function must return type bytea.
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 input function must be created first, then the output function (and the
binary I/O functions if wanted), and finally the data type. PostgreSQL
will first see the name of the new data type as the return type of the
input function. It will create a ``shell'' type, which is simply a
placeholder entry in the system catalog, and link the input function
definition to the shell type. Similarly the other functions will be linked
to the (now already existing) shell type. Finally, CREATE TYPE replaces the
shell entry with a complete type definition, and the new type can be used.
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 may not 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 may 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 may 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 may 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 base data type is created, PostgreSQL automatically
creates an associated array type, whose name consists of the base type's
name prepended with an underscore. The parser understands this naming
convention, and translates requests for columns of type foo[] into requests
for type _foo. The implicitly-created array type is variable length and
uses the built-in input and output functions array_in and array_out.
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 name allows its constituent char elements to be
accessed this way. A 2-D point type could allow its two component numbers
to be accessed like 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.
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.
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.
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
User-defined type names cannot begin with the underscore character (_) and
can only be 62 characters long (or in general NAMEDATALEN - 2, rather than
the NAMEDATALEN - 1 characters allowed for other names). Type names
beginning with underscore are reserved for internally-created array type
names.
In PostgreSQL versions before 7.3, it was customary to avoid creating a
shell type 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 functions declared using opaque, but it
will issue a notice and change the function's declaration 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 the base data type box and then uses the type in a
table definition:
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 the
chapter called ``Extending SQL'' in the documentation.
COMPATIBILITY
This CREATE TYPE command is a PostgreSQL extension. There is a CREATE TYPE
statement in SQL99 that is rather different in detail.
SEE ALSO
CREATE FUNCTION [create_function(5)], DROP TYPE [drop_type(l)]
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