PostgreSQL 8.0.3 Documentation
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# CREATE TYPE

## Name

CREATE TYPE -- define a new data type

## Synopsis

    CREATE TYPE name AS
        ( attribute_name data_type [, ... ] )

    CREATE TYPE name (
        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 ]
    )

## Description

`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.)

### Composite Types

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.

### Base Types

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 element type's OID in case this is an array type (or the type's own OID for a composite type), and the third is the `typmod` of the destination column, if known (-1 will be passed if not). The input function 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 OID for array types or the type OID for composite types. The output function should return type `cstring`.

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 two arguments of types `internal` and `oid`. It must return a value of the data type itself. (The first argument is a pointer to a `StringInfo` buffer holding the received byte string; the optional second argument is the element type OID in case this is an array type, or the type's own OID for a composite type.) 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 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 OID for array types or the type OID for composite types. The send function must return type `bytea`.

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 input function must be created first, then the output function (and the binary I/O functions if wanted), and finally 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 the system catalog, and link the input function definition to the shell type. Similarly the other functions 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.

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.

### 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 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.

## Parameters

`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.

## 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.

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 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 functions declared using `opaque`, but it will issue a notice and change the function's declaration to use the correct types.

## Examples

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 (
        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 31.11.

## Compatibility

This `CREATE TYPE` command is a PostgreSQL extension. There is a `CREATE TYPE` statement in SQL:1999 and later that is rather different in detail.

## See Also

CREATE FUNCTION, DROP TYPE, ALTER TYPE

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