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RedHat 9 (Linux i386) - man page for create_type (redhat section 7)

CREATE TYPE(7)				   SQL Commands 			   CREATE TYPE(7)

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  CUR-
       RENT_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 reg-
       istration of two functions (using CREATE FUNCTION) before defining the type. The represen-
       tation  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  func-
       tions  defined  for  the type. Naturally, output_function performs the reverse transforma-
       tion. 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  inte-
       ger,  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 ELE-
       MENT 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 capabil-
       ity: 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 general-
       ized  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 inter-
       nally-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 [create_function(7)], DROP TYPE	[drop_type(l)],  PostgreSQL  Programmer's
       Guide

SQL - Language Statements		    2002-11-22				   CREATE TYPE(7)


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