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CREATE
NAME
CREATE TYPE - define a new data type
SYNOPSIS
CREATE TYPE typename ( INPUT = input_function, OUTPUT = output_function
, INTERNALLENGTH = { internallength | VARIABLE }
[ , DEFAULT = default ]
[ , ELEMENT = element ] [ , DELIMITER = delimiter ]
[ , PASSEDBYVALUE ]
[ , ALIGNMENT = alignment ]
[ , STORAGE = storage ]
)
CREATE TYPE typename AS
( column_name data_type [, ... ] )
INPUTS
typename
The name (optionally schema-qualified) of a type to be created.
internallength
A literal value, which specifies the internal length of the new type.
input_function
The name of a function, created by CREATE FUNCTION, which converts
data from its external form to the type's internal form.
output_function
The name of a function, created by CREATE FUNCTION, which converts
data from its internal form to a form suitable for display.
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.
default
The default value for the data type. Usually this is omitted, so that
the default is NULL.
alignment
Storage alignment requirement of the data type. If specified, must be
char, int2, int4, or double; the default is int4.
storage
Storage technique for the data type. If specified, must be plain,
external, extended, or main; the default is plain.
column_name
The name of a column of the composite type.
data_type
The name of an existing data type.
OUTPUTS
CREATE TYPE
Message returned if the type is successfully created.
DESCRIPTION
CREATE TYPE allows the user to register a new data type with PostgreSQL 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 one at the front of the
search path; see 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, type names also must not conflict with table
names in the same schema.)
BASE TYPES
The first form of CREATE TYPE creates a new base type (scalar type). It
requires the registration of two functions (using CREATE FUNCTION) before
defining the type. The representation of a new base type is determined by
input_function, which converts the type's external representation to an
internal representation usable by the operators and functions defined for
the type. Naturally, 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, int4. (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.) It 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.
You should at this point be wondering how the input and output functions
can be declared to have results or inputs 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, then 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 pg_type, and link the input function
definition to the shell type. Similarly the output function 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.
Note: 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 pseudo-type OPAQUE. The cstring
inputs 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.
New 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 an
integer giving the total length of this value of the type.
To indicate that a type is an array, specify the type of the array elements
using the ELEMENT keyword. 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.
A default value may be specified, in case a user wants columns of the data
type to default to something other than NULL. Specify the default with the
DEFAULT keyword. (Such a default may be overridden by an explicit DEFAULT
clause attached to a particular column.)
The optional flag, PASSEDBYVALUE, indicates that values of this data type
are passed by value rather than by reference. Note that you may not pass by
value types whose internal representation is longer than the width of the
Datum type (four bytes on most machines, eight bytes on a few).
The alignment keyword 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 keyword allows selection of storage strategies for variable-
length data types (only plain is allowed for fixed-length types). plain
disables TOAST for the data type: it will always be stored in-line and not
compressed. extended gives full TOAST capability: 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 method 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.)
COMPOSITE TYPES
The second form of CREATE TYPE creates a composite type. The composite
type is specified by a list of column 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.
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 is there 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 N identical things, and you want to
allow the N 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 chars to be accessed this way. A
2-D point type could allow its two component floats 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 N 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.
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.
EXAMPLES
This example creates the box data type and then uses the type in a table
definition:
CREATE TYPE box (INTERNALLENGTH = 16,
INPUT = my_procedure_1, OUTPUT = my_procedure_2);
CREATE TABLE myboxes (id INT4, description box);
If box's internal structure were an array of four float4s, we might instead
say
CREATE TYPE box (INTERNALLENGTH = 16,
INPUT = my_procedure_1, OUTPUT = my_procedure_2,
ELEMENT = float4);
which would allow a box value's component floats 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 int4, obj bigobj);
This example creates a composite type and uses it in a table function
definition:
CREATE TYPE compfoo AS (f1 int, f2 text);
CREATE FUNCTION getfoo() RETURNS SETOF compfoo AS 'SELECT fooid, fooname FROM foo' LANGUAGE SQL;
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)], PostgreSQL
Programmer's Guide
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