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CREATE TYPE(7) SQL Commands CREATE TYPE(7)
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_out-
put_function and analyze_function are optional. Generally 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 opera-
tors 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 func-
tion 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 func-
tion can be declared as taking one argument of type internal, or as taking three arguments of types internal, oid, inte-
ger. 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_func-
tion 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 defini-
tion, 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_modi-
fier_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_modi-
fier_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 overrid-
den 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 Post-
greSQL. 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 giv-
ing 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 align-
ment 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 com-
press 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 situa-
tions. 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 documen-
tation. 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 internal-
length, passedbyvalue, alignment, and storage are copied from that type, unless overridden by explicit specifica-
tion 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 gener-
ated 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 cre-
ate 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 func-
tion 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 [cre-
ate_domain(7)]
SQL - Language Statements 2011-09-22 CREATE TYPE(7)
