CREATE TYPE — define a new data type
CREATE TYPEname
AS (attribute_name
data_type
[, ... ] ) CREATE TYPEname
( INPUT =input_function
, OUTPUT =output_function
[ , RECEIVE =receive_function
] [ , SEND =send_function
] [ , ANALYZE =analyze_function
] [ , INTERNALLENGTH = {internallength
| VARIABLE } ] [ , PASSEDBYVALUE ] [ , ALIGNMENT =alignment
] [ , STORAGE =storage
] [ , DEFAULT =default
] [ , ELEMENT =element
] [ , DELIMITER =delimiter
] ) CREATE TYPEname
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.)
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.
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
,
send_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 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 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
may 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 may 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
may 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
, with no additional parameters. Then the
I/O functions can be defined referencing the shell type. Finally,
name
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 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 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.
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.
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.
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
.
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.
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.
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 creator 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 8.2, the syntax
CREATE TYPE
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 may 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.
name
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.
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; 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 Section 33.11, “User-Defined Types”.