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

       CREATE CAST - define a new cast

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

       CREATE CAST (sourcetype AS targettype)

       CREATE CAST (sourcetype AS targettype)
           WITH INOUT

       CREATE  CAST defines a new cast. A cast specifies how to perform a conversion between two data types. For exam-

       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 invok-
       ing 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  per-
       formed 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  gener-
       ally 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 invok-
       able only for information-preserving transformations between types in the same general type category. For exam-
       ple, 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.)

              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 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.

              Indicates  that  the  source  type is binary-coercible to the target type, so no function is required to
              perform the cast.

              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.

              Indicates that the cast can be invoked implicitly in assignment contexts.

              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  repre-
       sents  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.

       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 declar-
       ing your own cast to replace an automatic cast, but usually the only reason to do so is if you want the conver-
       sion  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 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 implemen-
       tation functions after the target data type. Many users are used to being able to cast data types using a func-
       tion-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 func-
              tion-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.

       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.)

       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.

       CREATE FUNCTION [create_function(7)], CREATE TYPE [create_type(7)], DROP CAST [drop_cast(7)]

SQL - Language Statements         2014-02-17                    CREATE CAST(7)