create_cast man page on UnixWare

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       CREATE CAST - define a new cast

       CREATE CAST (sourcetype AS targettype)
	   WITH FUNCTION funcname (argtypes)

       CREATE CAST (sourcetype AS targettype)

       CREATE  CAST defines a new cast. A cast specifies how to perform a con‐
       version between two data types. For example,

       SELECT CAST(42 AS text);

       converts the integer constant 42 to type text by invoking a  previously
       specified  function,  in this case text(int4). (If no suitable cast has
       been defined, the conversion fails.)

       Two types may be binary compatible, which means that they can  be  con‐
       verted  into  one  another  ``for free'' without invoking any function.
       This requires that corresponding values use the same internal represen‐
       tation. For instance, the types text and varchar are binary compatible.

       By  default,  a	cast  can be invoked only by an explicit cast request,
       that is an explicit CAST(x AS typename) or x::typename construct.

       If the cast is marked AS ASSIGNMENT then it can be  invoked  implicitly
       when  assigning a value to a column of the target data type.  For exam‐
       ple, supposing that foo.f1 is a column of type text, then

       INSERT INTO foo (f1) VALUES (42);

       will be allowed if the cast from type integer to type text is marked AS
       ASSIGNMENT,  otherwise not.  (We generally use the term assignment cast
       to describe this kind of cast.)

       If the cast is marked AS IMPLICIT then it can be invoked implicitly  in
       any  context,  whether  assignment  or internally in an expression. For
       example, since || takes text operands,

       SELECT 'The time is ' || now();

       will be allowed only if the cast from type timestamp to text is	marked
       AS  IMPLICIT.  Otherwise it will be necessary to write the cast explic‐
       itly, for example

       SELECT 'The time is ' || CAST(now() AS text);

       (We generally use the term implicit  cast  to  describe	this  kind  of

       It is wise to be conservative about marking casts as implicit. An over‐
       abundance of implicit casting paths can cause PostgreSQL to choose sur‐
       prising	interpretations	 of  commands, or to be unable to resolve com‐
       mands at all because there are  multiple	 possible  interpretations.  A
       good  rule  of  thumb  is  to make a cast implicitly invokable only for
       information-preserving transformations between types in the  same  gen‐
       eral type category. For example, the cast from int2 to int4 can reason‐
       ably be implicit, but the cast from float8 to int4 should  probably  be
       assignment-only.	 Cross-type-category  casts, such as text to int4, are
       best made explicit-only.

       To be able to create a cast, you must own the source or the target data
       type. To create a binary-compatible cast, you must be superuser.	 (This
       restriction is made because an erroneous binary-compatible cast conver‐
       sion can easily crash the server.)

	      The name of the source data type of the cast.

	      The name of the target data type of the cast.

	      The  function used to perform the cast. The function name may be
	      schema-qualified. If it is not, the function will be  looked  up
	      in  the schema search path. The function's result data type must
	      match the target type of the cast. Its arguments	are  discussed

	      Indicates	 that  the  source type and the target type are binary
	      compatible, so no function is required to perform the cast.

	      Indicates that the cast may be invoked implicitly in  assignment

	      Indicates	 that  the  cast may be invoked implicitly in any con‐

       Cast implementation functions may have one  to  three  arguments.   The
       first  argument	type must be identical to the cast's source type.  The
       second argument, if present, must be type integer; it receives the type
       modifier	 associated with the destination type, or -1 if there is none.
       The third argument, if present, must be type boolean; it receives  true
       if  the cast is an explicit cast, false otherwise.  (Bizarrely, the SQL
       spec demands different behaviors for explicit  and  implicit  casts  in
       some cases. This argument is supplied for functions that must implement
       such casts. It is not recommended that you design your own  data	 types
       so that this matters.)

       Ordinarily  a  cast  must  have different source and target data types.
       However, it is allowed to declare a cast with identical source and tar‐
       get  types  if it has a cast implementation function with more than one
       argument. This is used to represent type-specific length coercion func‐
       tions  in  the  system catalogs. The named function is used to coerce a
       value of the type to the type modifier value given by its second	 argu‐
       ment.  (Since  the grammar presently permits only certain built-in data
       types to have type modifiers, this feature  is  of  no  use  for	 user-
       defined target types, but we mention it for completeness.)

       When  a	cast has different source and target types and a function that
       takes more than one argument, it represents converting from one type to
       another	and  applying a length coercion in a single step. When no such
       entry is available, coercion to	a  type	 that  uses  a	type  modifier
       involves	 two  steps, one to convert between data types and a second to
       apply the modifier.

       Use DROP CAST [drop_cast(5)] to remove user-defined casts.

       Remember that if you want to be able to convert	types  both  ways  you
       need to declare casts both ways explicitly.

       Prior  to  PostgreSQL  7.3,  every function that had the same name as a
       data type, returned that data type, and took one argument of a  differ‐
       ent  type  was automatically a cast function.  This convention has been
       abandoned in face of the introduction of schemas and to be able to rep‐
       resent  binary  compatible  casts  in the system catalogs. The built-in
       cast functions still follow this naming scheme, but  they  have	to  be
       shown as casts in the system catalog pg_cast as well.

       While  not required, it is recommended that you continue to follow this
       old convention of naming cast implementation functions after the target
       data type. Many users are used to being able to cast data types using a
       function-style notation, that is typename(x). This notation is in  fact
       nothing	more nor less than a call of the cast implementation function;
       it is not specially treated as a cast. If your conversion functions are
       not  named  to  support	this  convention  then you will have surprised
       users.  Since PostgreSQL allows overloading of the same	function  name
       with  different argument types, there is no difficulty in having multi‐
       ple conversion functions from different types that all use  the	target
       type's name.

	      Note:  There  is one small lie in the preceding paragraph: there
	      is still one case in which pg_cast will be used to  resolve  the
	      meaning of an apparent function call. If a function call name(x)
	      matches no actual function, but name is the name of a data  type
	      and pg_cast shows a binary-compatible cast to this type from the
	      type of x, then the call will be construed as an explicit	 cast.
	      This  exception  is  made so that binary-compatible casts can be
	      invoked using functional syntax, even though they lack any func‐

       To  create  a  cast  from  type	text  to  type int4 using the function

       CREATE CAST (text AS int4) WITH FUNCTION int4(text);

       (This cast is already predefined in the system.)

       The CREATE CAST command conforms to the SQL standard, except  that  SQL
       does not make provisions for binary-compatible types or extra arguments
       to implementation functions.  AS IMPLICIT is  a	PostgreSQL  extension,

       CREATE  FUNCTION	 [create_function(5)],	CREATE	TYPE [create_type(5)],
       DROP CAST [drop_cast(5)]

SQL - Language Statements	  2008-01-03			 CREATE CAST()
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