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CREATE TYPE

Updated at: Jul 15, 2020 GMT+08:00

Function

CREATE TYPE defines a new data type in the current database. The user who defines a new data type becomes its owner. Types are designed only for row-store tables.

Four types of data can be created by using CREATE TYPE: composite data, base data, a shell data, and enumerated data.

  • Composite types

    A composite type is specified by a list of attribute names and data types. If the data type of an attribute is collatable, the attribute's collation rule can also be specified. A composite type is essentially the same as the row type of a table. However, using CREATE TYPE avoids the need to create an actual table when only a type needs to be defined. In addition, a standalone composite type is useful, for example, as the parameter or return type of a function.

    To create a composite type, you must have the USAGE permission for all its attribute types.

  • Base types

    You can customize a new base type (scalar type). Generally, functions required for base types must be coded in C or another low-level language.

  • Shell types

    A shell type is simply a placeholder for a type to be defined later. It can be created by delivering CREATE TYPE with no parameters except for a type name. Shell types are needed as forward references when base types are created.

  • Enumerated types

    An enumerated type is a list of enumerated values. Each value is a non-empty string with the maximum length of 64 bytes.

Precautions

If a schema name is given, the type will be created in the specified schema. Otherwise, it will be created in the current schema. A type name must be different from the name of any existing type or domain in the same schema. (Since tables have associated data types, a type name must also be different from the name of any existing table in the same schema.)

Syntax

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

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 ]
    [ , ANALYZE = analyze_function ]
    [ , INTERNALLENGTH = { internallength |
VARIABLE } ]
    [ , PASSEDBYVALUE ]
    [ , ALIGNMENT = alignment ]
  [ , STORAGE = storage ]
    [ , LIKE = like_type ]
    [ , CATEGORY = category ]
    [ , PREFERRED = preferred ]
    [ , DEFAULT = default ]
    [ , ELEMENT = element ]
    [ , DELIMITER = delimiter ]
    [ , COLLATABLE = collatable ]
)

CREATE TYPE name

CREATE TYPE name AS ENUM
    ( [ 'label' [, ... ] ] )

Parameter Description

Composite types

  • name

    Specifies the name of the type to be created. It can be schema-qualified.

  • attribute_name

    Specifies the name of an attribute (column) for the composite type.

  • data_type

    Specifies the name of an existing data type to become a column of the composite type.

  • collation

    Specifies the name of an existing collation rule to be associated with a column of the composite type.

Base types

When creating a base type, you can place parameters in any order. The input_function and output_function parameters are mandatory, and other parameters are optional.

  • input_function

    Specifies the name of a function that converts data from the external text format of a type to its internal format.

    An input function can be declared as taking one parameter of the cstring type or taking three parameters of the cstring, oid, and integer types.

    • The cstring-type parameter is the input text as a C string.
    • The oid-type parameter is the OID of the type (except for array types, where the parameter is the element type OID of an array type).
    • The integer-type parameter is typmod of the destination column, if known (-1 will be passed if not known).

    An input function must return a value of the data type itself. Generally, an input function must be declared as STRICT. If it is not, it will be called with a NULL parameter coming first when the system reads a NULL input value. In this case, the function must still return NULL unless an error raises. (This mechanism is designed for supporting domain input functions, which may need to reject NULL input values.)

    Input and output functions can be declared to have the results or parameters of a new type because they have to be created before the new type is created. The new type should first be defined as a shell type, which is a placeholder type that has no attributes except a name and an owner. This can be done by delivering the CREATE TYPE name statement, with no additional parameters. Then, the C I/O functions can be defined as referencing the shell type. Finally, CREATE TYPE with a full definition replaces the shell type with a complete, valid type definition. After that, the new type can be used normally.

  • output_function

    Specifies the name of a function that converts data from the internal format of a type to its external text format.

    An output function must be declared as taking one parameter of a new data type. It must return data of the cstring type. Output functions are not invoked for NULL values.

  • receive_function

    (Optional) Specifies the name of a function that converts data from the external binary format of a type to its internal format.

    If this function is not used, the type cannot participate in binary input. It costs lower to convert the binary format to the internal format, more portable. (For example, the standard integer data types use the network byte order as an external binary representation, whereas the internal representation is in the machine's native byte order.) This function should perform adequate checks to ensure a valid value.

    Also, this function can be declared as taking one parameter of the internal type or taking three parameters of the internal, oid, and integer types.

    • The internal-type parameter is a pointer to a StringInfo buffer holding received byte strings.
    • The oid- and integer-type parameters are the same as those of the text input function.

    A receive function must return a value of the data type itself. Generally, a receive function must be declared as STRICT. If it is not, it will be called with a NULL parameter coming first when the system reads a NULL input value. In this case, the function must still return NULL unless an error raises. (This mechanism is designed for supporting domain receive functions, which may need to reject NULL input values.)

  • send_function

    (Optional) Specifies the name of a function that converts data from the internal format of a type to its external binary format.

    If this function is not used, the type cannot participate in binary output. A send function must be declared as taking one parameter of a new data type. It must return data of the bytea type. Send functions are not invoked for NULL values.

  • type_modifier_input_function

    (Optional) Specifies the name of a function that converts an array of modifiers for a type to its internal format.

  • type_modifier_output_function

    (Optional) Specifies the name of a function that converts the internal format of modifiers for a type to its external text format.

    type_modifier_input_function and type_modifier_output_function are needed if a type supports modifiers, that is, optional constraints attached to a type declaration, such as char(5) or numeric(30,2). DWS allows user-defined types to take one or more simple constants or identifiers as modifiers. However, this information must be capable of being packed into a single non-negative integer value for storage in system catalogs. Declared modifiers are passed to type_modifier_input_function in the cstring array format. The parameter must check values for validity, throwing an error if they are wrong. If they are correct, the parameter will return a single non-negative integer value, which will be stored as typmod in a column. If the type does not have type_modifier_input_function, type modifiers will be rejected. type_modifier_output_function converts the internal integer typmod value back to a correct format for user display. It must return a cstring value, which is the exact string appending to the type name. For example, a numeric function may return (30,2). If the default display format is enclosing a stored typmod integer value in parentheses, you can omit type_modifier_output_function.

  • analyze_function

    (Optional) Specifies the name of a function that performs statistical analysis for a data type.

    By default, if there is a default B-tree operator class for a type, ANALYZE will attempt to gather statistics by using the "equals" and "less-than" operators of the type. This behavior is inappropriate for non-scalar types, and can be overridden by specifying a custom analysis function. The analysis function must be declared to take one parameter of the internal type and return a boolean result.

  • internallength

    (Optional) Specifies a numeric constant for specifying the length in bytes of the internal representation of a new type. By default, it is variable-length.

    Although the details of the new type's internal representation are only known to I/O functions and other functions that you create to work with the type, there are still some attributes of the internal representation that must be declared to DWS. The most important one is internallength. Base data types can be fixed-length (when internallength is a positive integer) or variable-length (when internallength is set 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. internallength defines the total length.

  • PASSEDBYVALUE

    (Optional) Specifies that values of a data type are passed by value, rather than by reference. Types passed by value must be fixed-length, and their internal representation cannot be larger than the size of the Datum type (4 bytes on some machines, and 8 bytes on others).

  • alignment

    (Optional) Specifies the storage alignment required for a data type. It supports values char, int2, int4, and double. The default value is int4.

    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 must contain an int4 value as their first component.

  • storage

    (Optional) Specifies the storage strategy for a data type.

    It supports values plain, external, extended, and main. The default value is plain.

    • plain specifies that data of a type will always be stored in-line and not compressed. (Only plain is allowed for fixed-length types.)
    • extended specifies that the system will first try to compress a long data value and will then move the value out of the main table row if it is still too long.
    • external allows a value to be moved out of the main table, but the system will not try to compress it.
    • main allows for compression, but discourages moving a value out of the main table. (Data items with this storage strategy might still be moved out of the main table if there is no other way to make a row fit. However, they will be kept in the main table preferentially over extended and external items.)

      All storage values except plain imply that the functions of the data type can handle values that have been toasted. A given value merely determines the default TOAST storage strategy for columns of a toastable data type. Users can choose other strategies for individual columns by using ALTER TABLE SET STORAGE.

  • like_type

    (Optional) Specifies the name of an existing data type that has the same representation as a new type. The values of internallength, passedbyvalue, alignment, and storage are copied from this type, unless they are overridden by explicit specifications elsewhere in the CREATE TYPE command.

    Specifying representation in this way is especially useful when the low-level implementation of a new type references an existing type.

  • category

    (Optional) Specifies the category code (a single ASCII character) for a type. The default value is U for a user-defined type. You can also choose other ASCII characters to create custom categories.

  • preferred

    (Optional) Specifies whether a type is preferred within its type category. If it is, the value will be TRUE, else FALSE. The default value is FALSE. Be cautious when creating a new preferred type within an existing type category because this could cause great changes in behavior.

    The category and preferred parameters can be used to help determine which implicit cast excels 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. If this rule is helpful in resolving overloaded functions or operators, the parser will prefer casting to preferred types (but only from other types within the same category). For types that have no implicit casts to or from any other types, it is sufficient to leave these parameters at their default values. However, for a group of types that have implicit casts, 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 helpful in adding a user-defined type to an existing built-in category, such as the numeric or string type. However, you can also create new entirely-user-defined type categories. Select any ASCII character other than an uppercase letter to name such a category.

  • default

    (Optional) Specifies the default value for a data type. If this parameter is omitted, the default value will be NULL.

    A default value can be specified if you expect the columns of a data type to default to something other than the NULL value. You can also specify a default value using the DEFAULT keyword. (Such a default value can be overridden by an explicit DEFAULT clause attached to a particular column.)

  • element

    (Optional) Specifies the type of an array element when an array type is created. For example, to define an array of 4-byte integers (int4), set ELEMENT to int4.

  • delimiter

    (Optional) Specifies the delimiter character to be used between values in arrays made of a type.

    delimiter can be set to a specific character. The default delimiter is a comma (,). Note that a delimiter is associated with the array element type, instead of the array type itself.

  • collatable

    (Optional) Specifies whether a type's operations can use collation information. If they can, the value will be TRUE, else FALSE (default).

    If collatable is TRUE, column definitions and expressions of a type may carry collation information by using the COLLATE clause. It is the implementations of functions operating on the type that actually use the collation information. This use cannot be achieved merely by marking the type collatable.

  • lable

    (Optional) Specifies a text label associated with an enumerated value. It is a non-empty string of up to 64 characters.

Whenever a user-defined type is created, DWS automatically creates an associated array type whose name consists of the element type name prepended with an underscore (_).

Examples

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-- Create a composite type, create a table, insert data, and make a query:
CREATE TYPE compfoo AS (f1 int, f2 text);
CREATE TABLE t1_compfoo(a int, b compfoo);
CREATE TABLE t2_compfoo(a int, b compfoo);
INSERT INTO t1_compfoo values(1,(1,'demo'));
INSERT INTO t2_compfoo select * from t1_typ5;
SELECT (b).f1 FROM t1_compfoo;
SELECT * FROM t1_compfoo t1 join t2_compfoo t2 on (t1.b).f1=(t1.b).f1;

-- Rename the data type:
ALTER TYPE compfoo RENAME TO compfoo1;

-- Change the owner of the user-defined type compfoo1 to usr1:
CREATE USER usr1 PASSWORD 'Bigdata123@';
ALTER TYPE compfoo1 OWNER TO usr1;

-- Change the schema of the user-defined type compfoo1 to usr1:
ALTER TYPE compfoo1 SET SCHEMA usr1;

Add a new attribute to the data type:
ALTER TYPE usr1.compfoo1 ADD ATTRIBUTE f3 int;

Delete the compfoo1 type:
DROP TYPE usr1.compfoo1 cascade;

Delete related tables and users:
DROP TABLE t1_compfoo;
DROP TABLE t2_compfoo;
DROP SCHEMA usr1;
DROP USER usr1;

-- Create an enumerated type.
CREATE TYPE bugstatus AS ENUM ('create', 'modify', 'closed');

-- Add a label.
ALTER TYPE bugstatus ADD VALUE IF NOT EXISTS 'regress' BEFORE 'closed';

-- Rename a label.
ALTER TYPE bugstatus RENAME VALUE 'create' TO 'new';

-- Compile the .so file and create a shell type:
CREATE TYPE complex;
-- This statement creates a placeholder for the type to be defined so that the type can be referenced when its I/O functions are defined. Then, you can define I/O functions. Note that the functions must be declared to take the NOT FENCED mode during creation.
CREATE FUNCTION
complex_in(cstring)
    RETURNS complex
    AS 'filename'
    LANGUAGE C IMMUTABLE STRICT not fenced;

CREATE FUNCTION
complex_out(complex)
    RETURNS cstring
    AS 'filename'
    LANGUAGE C IMMUTABLE STRICT not fenced;

CREATE FUNCTION
complex_recv(internal)
  
RETURNS complex
  
AS 'filename'
  
LANGUAGE C IMMUTABLE STRICT not fenced;

CREATE FUNCTION
complex_send(complex)
  
RETURNS bytea
  
AS 'filename'
  
LANGUAGE C IMMUTABLE STRICT not fenced;
-- Finally, provide a complete definition of the data type:
CREATE TYPE complex (
  
internallength = 16,
  
input = complex_in,
  
output = complex_out,
  
receive = complex_recv,
  
send = complex_send,
  
alignment = double
);

The C functions corresponding to the input, output, receive, and send functions are defined as follows:

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-- Define a structure body Complex:
typedef struct Complex {
    double      x;
    double      y;
} Complex;

-- Define an input function:
PG_FUNCTION_INFO_V1(complex_in);

Datum
complex_in(PG_FUNCTION_ARGS)
{
    char       *str = PG_GETARG_CSTRING(0);
    double      x,
                y;
    Complex    *result;

    if (sscanf(str, " ( %lf , %lf )", &x, &y) != 2)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid input syntax for complex: \"%s\"",
                        str)));

    result = (Complex *) palloc(sizeof(Complex));
    result->x = x;
    result->y = y;
    PG_RETURN_POINTER(result);
}

-- Define an output function:
PG_FUNCTION_INFO_V1(complex_out);

Datum
complex_out(PG_FUNCTION_ARGS)
{
        Complex    *complex = (Complex *) PG_GETARG_POINTER(0);
        char       *result;

        result = (char *) palloc(100);
        snprintf(result, 100, "(%g,%g)", complex->x, complex->y);
        PG_RETURN_CSTRING(result);
}

-- Define a receive function:
PG_FUNCTION_INFO_V1(complex_recv);

Datum
complex_recv(PG_FUNCTION_ARGS)
{
    StringInfo  buf = (StringInfo) PG_GETARG_POINTER(0);
    Complex    *result;

    result = (Complex *) palloc(sizeof(Complex));
    result->x = pq_getmsgfloat8(buf);
    result->y = pq_getmsgfloat8(buf);
    PG_RETURN_POINTER(result);
}

-- Define a send function:
PG_FUNCTION_INFO_V1(complex_send);

Datum
complex_send(PG_FUNCTION_ARGS)
{
    Complex    *complex = (Complex *) PG_GETARG_POINTER(0);
    StringInfoData buf;

    pq_begintypsend(&buf);
    pq_sendfloat8(&buf, complex->x);
    pq_sendfloat8(&buf, complex->y);
    PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}

Helpful Links

ALTER TYPE, DROP TYPE

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