CREATE TYPECREATE TYPE7SQL - Language StatementsCREATE TYPEdefine a new data type
CREATE TYPE name AS
( [ attribute_namedata_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 nameDescriptionCREATE 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
The second form of CREATE TYPE creates an enumerated
(enum) type, as described in .
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.)
Range Types
The third form of CREATE TYPE creates a new
range type, as described in .
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 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 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
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 ; 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 and .
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 . 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 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
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
option, if the system makes the correct array type automatically.
The main case where it's useful to use 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 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 handler
function raw_array_subscript_handler, which is
used automatically if you specify for a
fixed-length type without also writing .
When specifying a custom function, it is
not necessary to specify unless
the handler function needs to
consult typelem to find out what to return.
Be aware that specifying 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.
Parametersname
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
. 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
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 .
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