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CREATE CAST

   CREATE CAST — define a new cast

Synopsis

CREATE CAST (source_type AS target_type)
    WITH FUNCTION function_name [ (argument_type [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (source_type AS target_type)
    WITHOUT FUNCTION
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (source_type AS target_type)
    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 by 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 resulting string to
   the input function of the target data type. In many common cases, this
   feature avoids the need to write a separate cast function for
   conversion. An I/O conversion cast acts the same as a regular
   function-based cast; only the implementation is different.

   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 for more
   information.

   To be able to create a cast, you must own the source or the target data
   type and have USAGE privilege on the other 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

   source_type
          The name of the source data type of the cast.

   target_type
          The name of the target data type of the cast.

   function_name[(argument_type [, ...])]
          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. If no argument list is specified, the function name must
          be unique in its schema.

   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 resulting string 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 supports 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 cast steps, one to convert between data types and a second
   to apply the modifier.

   A cast to or from a domain type currently has no effect. Casting to or
   from a domain uses the casts associated with its underlying type.

Notes

   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.

   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.)

   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.

Note

   There is also an exception to the exception: I/O conversion casts from
   composite types to string types cannot be invoked using functional
   syntax, but must be written in explicit cast syntax (either CAST or ::
   notation). This exception was added because after the introduction of
   automatically-provided I/O conversion casts, it was found too easy to
   accidentally invoke such a cast when a function or column reference was
   intended.

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 TYPE, DROP CAST