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 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
       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, 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 avail-
       able 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 commands, 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 categories and pre-
	      ferred 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 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 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 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 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 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 implementation 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 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 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  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
       represent 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 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(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  argu-
       ments 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)