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

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

       CREATE CAST - define a user-defined cast

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

       CREATE CAST (sourcetype AS targettype)

       CREATE  CAST  defines a new cast. A cast specifies how to perform a conversion 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 converted into one
       another ``for free'' without invoking any function. This requires that corresponding  val-
       ues  use  the  same  internal representation. 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), x::typename, or typename(x) construct.

       If  the cast is marked AS ASSIGNMENT then it can be invoked implicitly when assigning 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. For example, since || takes text argu-

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

       will be allowed only if the cast from type timestamp to text is marked AS IMPLICIT. Other-
       wise it will be necessary to write the cast explicitly, for example

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

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

       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.

       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 erro-
       neous binary-compatible cast conversion can easily crash the server).  "PARAMETERS"

	      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 path. The argument type must be
	      identical to the source type, the result data type must match the  target  type  of
	      the cast. Cast functions must be marked immutable or stable.

	      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 contexts.

	      Indicates that the cast may be invoked implicitly in any context.

       Use DROP CAST 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 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 compatible casts in the catalogs. (The	built-in  cast	functions
       still follow this naming scheme, but they have to be shown as casts in pg_cast now.)

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

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

       (This cast is already predefined in the system.)

       The  CREATE CAST command conforms to SQL99, except that SQL99 does not make provisions for
       binary compatible types. AS IMPLICIT is a PostgreSQL extension, too.

       CREATE  FUNCTION   [create_function(7)],   CREATE   TYPE   [create_type(7)],   DROP   CAST
       [drop_cast(7)], PostgreSQL Programmer's Guide

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

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