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CREATE CAST(7)				   SQL Commands 			   CREATE CAST(7)

NAME
       CREATE CAST - define a new cast

SYNOPSIS
       CREATE CAST (sourcetype AS targettype)
	   WITH FUNCTION funcname (argtypes)
	   [ AS ASSIGNMENT | AS IMPLICIT ]

       CREATE CAST (sourcetype AS targettype)
	   WITHOUT FUNCTION
	   [ AS ASSIGNMENT | AS IMPLICIT ]

       CREATE CAST (sourcetype AS targettype)
	   WITH INOUT
	   [ AS ASSIGNMENT | AS IMPLICIT ]

DESCRIPTION
       CREATE  CAST  defines a new cast. A cast specifies how to perform a conversion between two
       data types. For example:

       SELECT CAST(42 AS float8);

       converts the integer constant 42 to type float8 by invoking a previously  specified  func-
       tion,  in  this	case  float8(int4). (If no suitable cast has been defined, the conversion
       fails.)

       Two types can be binary coercible, which means that the conversion can be performed  ``for
       free'' without invoking any function. This requires that corresponding values use the same
       internal representation. For instance, the types text and  varchar  are	binary	coercible
       both  ways.  Binary coercibility is not necessarily a symmetric relationship. For example,
       the cast from xml to text can be performed for free in the present implementation, but the
       reverse	direction  requires  a function that performs at least a syntax check. (Two types
       that are binary coercible both ways are also referred to as binary compatible.)

       You can define a cast as an I/O conversion cast using the WITH INOUT syntax. An	I/O  con-
       version	cast  is  performed  by invoking the output function of the source data type, and
       passing the result to the input function of the target data type.

       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 example, 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, other-
       wise 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. (We generally use the term implicit
       cast to describe this kind of cast.)  For example, consider this query:

       SELECT 2 + 4.0;

       The parser initially marks the constants as being of  type  integer  and  numeric  respec-
       tively.	There  is  no  integer	+ numeric operator in the system catalogs, but there is a
       numeric + numeric operator.  The query will therefore succeed if a cast	from  integer  to
       numeric	is  available  and  is marked AS IMPLICIT -- which in fact it is. The parser will
       apply the implicit cast and resolve the query as if it had been written

       SELECT CAST ( 2 AS numeric ) + 4.0;

       Now, the catalogs also provide a cast from numeric to integer. If that cast were marked AS
       IMPLICIT  --  which  it is not -- then the parser would be faced with choosing between the
       above interpretation and the alternative of casting the numeric constant  to  integer  and
       applying  the integer + integer operator. Lacking any knowledge of which choice to prefer,
       it would give up and declare the query ambiguous. The fact that only one of the two  casts
       is  implicit  is  the  way  in  which  we teach the parser to prefer resolution of a mixed
       numeric-and-integer expression as numeric; there is no built-in knowledge about that.

       It is wise to be conservative  about  marking  casts  as  implicit.  An	overabundance  of
       implicit  casting  paths can cause PostgreSQL to choose surprising interpretations of com-
       mands, or to be unable to resolve commands 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  general  type	category.
       For  example,  the  cast  from  int2 to int4 can reasonably 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.

	      Note:  Sometimes	it  is necessary for usability or standards-compliance reasons to
	      provide multiple implicit casts among a set of types, resulting in  ambiguity  that
	      cannot be avoided as above. The parser has a fallback heuristic based on type cate-
	      gories and preferred types that can help to provide desired behavior in such cases.
	      See CREATE TYPE [create_type(7)] for more information.

       To  be able to create a cast, you must own the source or the target data type. To create a
       binary-coercible cast, you must be superuser.  (This restriction is made because an  erro-
       neous binary-coercible cast conversion can easily crash the server.)

PARAMETERS
       sourcetype
	      The name of the source data type of the cast.

       targettype
	      The name of the target data type of the cast.

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

       WITHOUT FUNCTION
	      Indicates that the source type is binary-coercible to the target type, so no  func-
	      tion is required to perform the cast.

       WITH INOUT
	      Indicates that the cast is an I/O conversion cast, performed by invoking the output
	      function of the source data type, and passing the result to the input  function  of
	      the target data type.

       AS ASSIGNMENT
	      Indicates that the cast can be invoked implicitly in assignment contexts.

       AS IMPLICIT
	      Indicates that the cast can be invoked implicitly in any context.

       Cast  implementation  functions	can have one to three arguments.  The first argument type
       must be identical to or binary-coercible from the cast's source type. The second argument,
       if present, must be type integer; it receives the type modifier associated with the desti-
       nation 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
       standard 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.)

       The return type of a cast function must be identical to or binary-coercible to the  cast's
       target type.

       Ordinarily  a  cast  must  have	different  source  and target data types.  However, it is
       allowed to declare a cast with identical source and target types if it has a  cast  imple-
       mentation  function  with  more than one argument. This is used to represent type-specific
       length coercion functions 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 argument.

       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.

NOTES
       Use DROP CAST [drop_cast(7)] 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.

       It is normally not necessary to create casts between user-defined types and  the  standard
       string  types  (text, varchar, and char(n), as well as user-defined types that are defined
       to be in the string category). PostgreSQL provides  automatic  I/O  conversion  casts  for
       that. The automatic casts to string types are treated as assignment casts, while the auto-
       matic casts from string types are explicit-only. You can override this behavior by declar-
       ing your own cast to replace an automatic cast, but usually the only reason to do so is if
       you want the conversion to be more easily invokable than the standard  assignment-only  or
       explicit-only  setting.	Another possible reason is that you want the conversion to behave
       differently from the type's I/O function; but that is  sufficiently  surprising	that  you
       should  think  twice about whether it's a good idea. (A small number of the built-in types
       do indeed have different behaviors for conversions, mostly because of requirements of  the
       SQL standard.)

       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 different type was automatically a  cast  func-
       tion.  This convention has been abandoned in face of the introduction of schemas and to be
       able to represent binary-coercible casts in the system catalogs. The built-in  cast  func-
       tions  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  sup-
       port  this  convention  then you will have surprised users.  Since PostgreSQL allows over-
       loading of the same function name with different argument types, there is no difficulty in
       having  multiple  conversion functions from different types that all use the target type's
       name.

	      Note: Actually the preceding paragraph is  an  oversimplification:  there  are  two
	      cases  in which a function-call construct will be treated as a cast request without
	      having matched it to an actual function.	If  a  function  call  name(x)	does  not
	      exactly  match  any  existing  function,	but  name  is the name of a data type and
	      pg_cast provides a binary-coercible cast to this type from the type of x, then  the
	      call  will  be construed as a binary-coercible cast. This exception is made so that
	      binary-coercible casts can be invoked using functional  syntax,  even  though  they
	      lack  any function. Likewise, if there is no pg_cast entry but the cast would be to
	      or from a string type, the call will be construed as an I/O conversion  cast.  This
	      exception allows I/O conversion casts to be invoked using functional syntax.

EXAMPLES
       To  create  an  assignment cast from type bigint to type int4 using the function int4(big-
       int):

       CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;

       (This cast is already predefined in the system.)

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

SEE ALSO
       CREATE  FUNCTION   [create_function(7)],   CREATE   TYPE   [create_type(7)],   DROP   CAST
       [drop_cast(7)]

SQL - Language Statements		    2010-05-14				   CREATE CAST(7)
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