CREATE TYPE(l)SQL - Language Statements (2002-11-22CREATE TYPE(l)
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
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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
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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.)
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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
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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
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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(l)], DROP TYPE
[drop_type(l)], PostgreSQL Programmer's Guide
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