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CREATE TYPE

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, DROP TYPE, PostgreSQL Programmer's Guide

Comments


Feb. 20, 2003, 8:19 p.m.

Sometimes it is helpful to have a data type that is essentially an alias for another datatype.
e.g.
An snmp gauge data type based on the int8 data type.
The select statements provided show that the definitions are equivalent.

CREATE FUNCTION gauge_in(cstring)
RETURNS gauge
AS \'int8in\'
LANGUAGE INTERNAL IMMUTABLE WITH (isstrict);

select oid,* from pg_proc where proname = \'int8in\' or proname = \'gauge_in\';

CREATE FUNCTION gauge_out(gauge)
RETURNS cstring
AS \'int8out\'
LANGUAGE INTERNAL IMMUTABLE WITH (isstrict);

select oid,* from pg_proc where proname = \'int8out\' or proname = \'gauge_out\';

CREATE TYPE gauge (
INTERNALLENGTH = 8,
INPUT = gauge_in,
OUTPUT = gauge_out,
STORAGE = plain,
ALIGNMENT = double
);
select oid,* from pg_type where typname = \'int8\' or typname = \'gauge\';

CREATE FUNCTION to_int8(gauge)
RETURNS int8
AS \'int8up\'
LANGUAGE INTERNAL IMMUTABLE WITH (isstrict);

CREATE CAST (gauge AS int8) WITH FUNCTION to_int8(gauge) AS IMPLICIT;

CREATE FUNCTION to_gauge(int8)
RETURNS gauge
AS \'int8up\'
LANGUAGE INTERNAL IMMUTABLE WITH (isstrict);

CREATE CAST (int8 AS gauge) WITH FUNCTION to_gauge(int8) AS IMPLICIT;


June 12, 2003, 3:20 a.m.

Alias types are provided for using the DOMAIN commands.

A DOMAIN is a base type (like int8) with optional CHECK constraints applied to it.

CREATE DOMAIN gauge AS int8;

CREATE TABLE test (col gauge);

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