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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 }
    [ , EXTERNALLENGTH = { externallength | VARIABLE } ]
    [ , DEFAULT = default ]
    [ , ELEMENT = element ] [ , DELIMITER = delimiter ]
    [ , SEND = send_function ] [ , RECEIVE = receive_function ]
    [ , PASSEDBYVALUE ]
    [ , ALIGNMENT = alignment ]
    [ , STORAGE = storage ]
)
  

Inputs

typename

The name of a type to be created.

internallength

A literal value, which specifies the internal length of the new type.

externallength

A literal value, which specifies the external (displayed) 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.

send_function

The name of a function, created by CREATE FUNCTION, which converts data of this type into a form suitable for transmission to another machine.

receive_function

The name of a function, created by CREATE FUNCTION, which converts data of this type from a form suitable for transmission from another machine to internal form.

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.

Outputs

CREATE

Message returned if the type is successfully created.

Description

CREATE TYPE allows the user to register a new user data type with PostgreSQL for use in the current data base. The user who defines a type becomes its owner. typename is the name of the new type and must be unique within the types defined for this database.

CREATE TYPE 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 opaque, or as taking three arguments of types opaque, 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.) The output function may be declared as taking one argument of type opaque, or as taking two arguments of types opaque, OID. (The first argument is actually of the data type itself, but since the output function must be declared first, it's easier to declare it as accepting type opaque. The second argument is again the array element type for array 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.

The external representation length is similarly specified using the externallength keyword. (This value is not presently used, and is typically omitted, letting it default to VARIABLE.)

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 arguments send_function and receive_function are not currently used, and are usually omitted (allowing them to default to the output_function and input_function respectively). These functions may someday be resurrected for use in specifying machine-independent binary representations.

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.)

Array Types

Whenever a user-defined 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 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 30 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);

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
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