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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 con‐
       version between two data types. For example:

       SELECT CAST(42 AS float8);

       converts the integer constant 42 to type float8 by  invoking  a	previ‐
       ously  specified	 function,  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	 conversion  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 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. (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 respectively. 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	alter‐
       native  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 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.

	      Note:  Sometimes it is necessary for usability or standards-com‐
	      pliance 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 erroneous binary-coercible cast conver‐
       sion 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 function'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 function 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	 pass‐
	      ing 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  con‐
	      text.

       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 inte‐
       ger; 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 standard demands different behaviors
       for explicit and implicit casts in some cases. This  argument  is  sup‐
       plied  for  functions  that must implement such casts. It is not recom‐
       mended 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 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.

       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	 auto‐
       matic  casts to string types are treated as assignment casts, while the
       automatic casts from string types are explicit-only. You	 can  override
       this  behavior by declaring 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 suffi‐
       ciently surprising that you should think twice  about  whether  it's  a
       good idea. (A small number of the built-in types do indeed have differ‐
       ent 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  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-coercible 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: 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(bigint):

       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 provisions 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	  2013-04-02			CREATE CAST(7)
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