SQL Commands - CREATE TYPE - 《Greenplum Database v6.24 Documentation》

Synopsis
Description
Parameters
Notes
Examples
Compatibility
See Also

Defines a new data type.

Synopsis

CREATE TYPE <name> AS 
    ( <attribute_name> <data_type> [ COLLATE <collation> ] [, ... ] ] )
CREATE TYPE <name> AS ENUM 
    ( [ '<label>' [, ... ] ] )
CREATE TYPE <name> AS RANGE (
    SUBTYPE = <subtype>
    [ , SUBTYPE_OPCLASS = <subtype_operator_class> ]
    [ , COLLATION = <collation> ]
    [ , CANONICAL = <canonical_function> ]
    [ , SUBTYPE_DIFF = <subtype_diff_function> ]
)
CREATE TYPE <name> (
    INPUT = <input_function>,
    OUTPUT = <output_function>
    [, RECEIVE = <receive_function>]
    [, SEND = <send_function>]
    [, TYPMOD_IN = <type_modifier_input_function> ]
    [, TYPMOD_OUT = <type_modifier_output_function> ]
    [, INTERNALLENGTH = {<internallength> | VARIABLE}]
    [, PASSEDBYVALUE]
    [, ALIGNMENT = <alignment>]
    [, STORAGE = <storage>]
    [, LIKE = <like_type>
    [, CATEGORY = <category>]
    [, PREFERRED = <preferred>]
    [, DEFAULT = <default>]
    [, ELEMENT = <element>]
    [, DELIMITER = <delimiter>]
    [, COLLATABLE = <collatable>]
    [, COMPRESSTYPE = <compression_type>]
    [, COMPRESSLEVEL = <compression_level>]
    [, BLOCKSIZE = <blocksize>] )
CREATE TYPE <name>

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. The type name must also be distinct from the name of any existing table in the same schema.

There are five forms of CREATE TYPE, as shown in the syntax synopsis above. They respectively create a composite type, an enum type, a range type, a base type, or a shell type. The first four of these are discussed in turn below. A shell type is simply a placeholder for a type to be defined later; it is created by issuing CREATE TYPE with no parameters except for the type name. Shell types are needed as forward references when creating range types and base types, as discussed in those sections.

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. An attribute’s collation can be specified too, if its data type is collatable. A composite type 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, for example, as the argument or return type of a function.

To be able to create a composite type, you must have USAGE privilege on all attribute types.

Enumerated Types

The second form of CREATE TYPE creates an enumerated (ENUM) type, as described in Enumerated Types in the PostgreSQL documentation. ENUM types take a list of quoted labels, each of which must be less than NAMEDATALEN bytes long (64 in a standard build).

It is possible to create an enumerated type with zero labels, but such a type cannot be used to hold values before at least one label is added using ALTER TYPE.

Range Types

The third form of CREATE TYPE creates a new range type, as described in Range Types.

The range type’s subtype can be any type with an associated b-tree operator class (to determine the ordering of values for the range type). Normally the subtype’s default b-tree operator class is used to determine ordering; to use a non-default operator class, specify its name with subtype_opclass. If the subtype is collatable, and you want to use a non-default collation in the range’s ordering, specify the desired collation with the collation option.

The optional canonical function must take one argument of the range type being defined, and return a value of the same type. This is used to convert range values to a canonical form, when applicable. See Section Defining New Range Types for more information. Creating a canonical function is a bit tricky, since it must be defined before the range type can be declared. To do this, you must first create a shell type, which is a placeholder type that has no properties except a name and an owner. This is done by issuing the command CREATE TYPE name, with no additional parameters. Then the function can be declared using the shell type as argument and result, and finally the range type can be declared using the same name. This automatically replaces the shell type entry with a valid range type.

The optional <subtype_diff> function must take two values of the subtype type as argument, and return a double precision value representing the difference between the two given values. While this is optional, providing it allows much greater efficiency of GiST indexes on columns of the range type. See Defining New Range Types for more information.

Base Types

The fourth form of CREATE TYPE creates a new base type (scalar type). You must be a superuser to create a new base type. The parameters may appear in any order, not only that shown in the syntax, 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, type_modifier_input_function, type_modifier_output_function, and analyze_function are optional. Generally these functions have to be coded in C or another low-level language. In Greenplum Database, any function used to implement a data type must be defined as IMMUTABLE.

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 type’s own OID (except for array types, which instead receive their element type’s OID), and the third is the typmod of the destination column, if known (-1 will be passed if not). The input function must return a value of the data type itself. Usually, an input function should be declared STRICT; if it is not, it will be called with a NULL first parameter when reading a NULL input value. The function must still return NULL in this case, unless it raises an error. (This case is mainly meant to support domain input functions, which may need to reject NULL inputs.) The output function must be declared as taking one argument of the new data type. The output function must return type cstring. Output functions are not invoked for NULL values.

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 as taking three arguments of types internal, oid, integer. The first argument is a pointer to a StringInfo buffer holding the received byte string; the optional arguments are the same as for the text input function. The receive function must return a value of the data type itself. Usually, a receive function should be declared STRICT; if it is not, it will be called with a NULL first parameter when reading a NULL input value. The function must still return NULL in this case, unless it raises an error. (This case is mainly meant to support domain receive functions, which may need to reject NULL inputs.) 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 must be declared as taking one argument of the new data type. The send function must return type bytea. Send functions are not invoked for NULL values.

The optional type_modifier_input_function and type_modifier_output_function are required if the type supports modifiers. Modifiers are optional constraints attached to a type declaration, such as char(5) or numeric(30,2). While Greenplum Database allows user-defined types to take one or more simple constants or identifiers as modifiers, this information must fit into a single non-negative integer value for storage in the system catalogs. Greenplum Database passes the declared modifier(s) to the type_modifier_input_function in the form of a cstring array. The modifier input function must check the values for validity, throwing an error if they are incorrect. If the values are correct, the modifier input function returns a single non-negative integer value that Greenplum Database stores as the column typmod. Type modifiers are rejected if the type was not defined with a type_modifier_input_function. The type_modifier_output_function converts the internal integer typmod value back to the correct form for user display. The modifier output function must return a cstring value that is the exact string to append to the type name. For example, numeric‘s function might return (30,2). The type_modifier_output_function is optional. When not specified, the default display format is the stored typmod integer value enclosed in parentheses.

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 type should first be defined as a shell type, which is a placeholder type that has no properties except a name and an owner. This is done by issuing the command CREATE TYPE name, with no additional parameters. Then the I/O functions can be defined referencing the shell type. Finally, CREATE TYPE with a full definition replaces the shell entry with a complete, valid type definition, after which the new type can be used normally.

The like_type parameter provides an alternative method for specifying the basic representation properties of a data type: copy them from some existing type. The values internallength, passedbyvalue, alignment, and storage are copied from the named type. (It is possible, though usually undesirable, to override some of these values by specifying them along with the LIKE clause.) Specifying representation this way is especially useful when the low-level implementation of the new type “piggybacks” on an existing type in some fashion.

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 Greenplum Database. 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.

The category and preferred parameters can be used to help control which implicit cast Greenplum Database applies in ambiguous situations. Each data type belongs to a category named by a single ASCII character, and each type is either “preferred” or not within its category. The parser will prefer casting to preferred types (but only from other types within the same category) when this rule helps resolve overloaded functions or operators. For types that have no implicit casts to or from any other types, it is sufficient to retain the default settings. However, for a group of related types that have implicit casts, it is often helpful to mark them all as belonging to a category and select one or two of the “most general” types as being preferred within the category. The category parameter is especially useful when you add a user-defined type to an existing built-in category, such as the numeric or string types. It is also possible to create new entirely-user-defined type categories. Select any ASCII character other than an upper-case letter to name such a category.

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.

If the optional Boolean parameter collatable is true, column definitions and expressions of the type may carry collation information through use of the COLLATE clause. It is up to the implementations of the functions operating on the type to actually make use of the collation information; this does not happen automatically merely by marking the type collatable.

Array Types

Whenever a user-defined type is created, Greenplum Database automatically creates an associated array type, whose name consists of the element type’s name prepended with an underscore, and truncated if necessary to keep it less than NAMEDATALEN bytes long. (If the name so generated collides with an existing type name, the process is repeated until a non-colliding name is found.) This implicitly-created array type is variable length and uses the built-in input and output functions array_in and array_out. The array type tracks any changes in its element type’s owner or schema, and is dropped if the element type is.

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 point is represented as just two floating-point numbers, each can be accessed using 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.

collation

The name of an existing collation to be associated with a column of a composite type, or with a range type.

label

A string literal representing the textual label associated with one value of an enum type.

subtype

The name of the element type that the range type will represent ranges of.

subtype_operator_class

The name of a b-tree operator class for the subtype.

canonical_function

The name of the canonicalization function for the range type.

subtype_diff_function

The name of a difference function for the subtype.

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.

type_modifier_input_function

The name of a function that converts an array of modifier(s) for the type to internal form.

type_modifier_output_function

The name of a function that converts the internal form of the type’s modifier(s) to external textual form.

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. Must be one of char, int2, int4, or double. The default is int4.

storage

The storage strategy for the data type. Must be one of plain, external, extended, or main. The default is plain.

like_type

The name of an existing data type that the new type will have the same representation as. The values internallength, passedbyvalue, alignment, and storage, are copied from that type, unless overridden by explicit specification elsewhere in this CREATE TYPE command.

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.

Greenplum Database does not support adding storage options for row or composite types.

Storage options defined at the table- and column- level override the default storage options defined for a scalar type.

Because there are no restrictions on use of a data type once it’s been created, creating a base type or range type is tantamount to granting public execute permission on the functions mentioned in the type definition. (The creator of the type is therefore required to own these functions.) This is usually not an issue for the sorts of functions that are useful in a type definition. But you might want to think twice before designing a type in a way that would require ‘secret’ information to be used while converting it to or from external form.

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 enumerated type mood and uses it in a table definition.

CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
CREATE TABLE person (
    name text,
    current_mood mood
);
INSERT INTO person VALUES ('Moe', 'happy');
SELECT * FROM person WHERE current_mood = 'happy';
 name | current_mood 
------+--------------
 Moe  | happy
(1 row)

This example creates a range type:

CREATE TYPE float8_range AS RANGE (subtype = float8, subtype_diff = float8mi);

This example creates the base data type box and then uses the type in a table definition:

CREATE TYPE box;
CREATE FUNCTION my_box_in_function(cstring) RETURNS box AS 
... ;
CREATE FUNCTION my_box_out_function(box) RETURNS cstring AS 
... ;
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
    );

Compatibility

The first form of the CREATE TYPE command, which creates a composite type, conforms to the SQL standard. The other forms are Greenplum Database extensions. The CREATE TYPE statement in the SQL standard also defines other forms that are not implemented in Greenplum Database.

The ability to create a composite type with zero attributes is a Greenplum Database-specific deviation from the standard (analogous to the same case in CREATE TABLE).