CREATE_CAST man page on Oracle

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CREATE CAST(7)		PostgreSQL 9.2.7 Documentation		CREATE CAST(7)

       CREATE_CAST - define a new cast

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

       CREATE CAST (source_type AS target_type)

       CREATE CAST (source_type AS target_type)

       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.

	   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 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.)

	   The name of the source data type of the cast.

	   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.

	   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 resulting string to the input function of the target data type.

	   Indicates that the cast can be invoked implicitly in assignment

	   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

       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.

       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.

	   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.

	   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.

       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,


PostgreSQL 9.2.7		  2014-02-17			CREATE CAST(7)

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