Provided by: postgresql-client-8.3_8.3.4-2.2_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 ]

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  compatible,  which  means  that  they  can  be
       converted  into one another ‘‘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
       compatible.

       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.

       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  erroneous  binary-compatible   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 and the target type are binary
              compatible, so no function is required to perform the cast.

       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 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
       spec 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.)

       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)).  PostgreSQL
       will automatically handle a cast to a string type by invoking the other
       type’s output function, or conversely handle a cast from a string  type
       by  invoking  the  other  type’s  input  function. These automatically-
       provided casts are known as I/O conversion casts. I/O conversion  casts
       to  string  types are treated as assignment casts, while I/O conversion
       casts from string  types  are  explicit-only.  You  can  override  this
       behavior  by declaring your own cast to replace an I/O conversion 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 compatible 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-compatible cast to this type from  the
              type  of  x,  then  the  call  will  be  construed  as a binary-
              compatible  cast.  This  exception  is  made  so  that   binary-
              compatible  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 a cast from type bigint  to  type  int4  using  the  function
       int4(bigint):

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

       (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-compatible 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)]