Provided by: postgresql-client-8.4_8.4.11-1_i386 bug

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  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 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 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
       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
       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 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
       different 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  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)]