Ubuntu Manpage: CREATE_CAST - define a new cast

Provided by: postgresql-client-17_17.2-1_amd64

NAME

       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 (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.)

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.