CREATE CAST (sourcetype AS targettype) WITH FUNCTION funcname (argtypes) [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (sourcetype AS targettype) WITHOUT FUNCTION [ AS ASSIGNMENT | AS IMPLICIT ]
CREATE CAST defines a new cast. A cast specifies how to perform a conversion between two data types. For example,
SELECT CAST(42 AS text);
converts the integer constant 42 to type text by invoking a previously specified function, in this case text(int4). (If no suitable cast has been defined, the conversion fails.)
Two types may 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. For example, since || takes text operands,
SELECT 'The time is ' || now();
will be allowed only if the cast from type timestamp to text is marked AS IMPLICIT. Otherwise it will be necessary to write the cast explicitly, for example
SELECT 'The time is ' || CAST(now() AS text);
(We generally use the term implicit cast to describe this kind of cast.)
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.)
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 may 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 and the target type are binary compatible, so no function is required to perform the cast.
Indicates that the cast may be invoked implicitly in assignment contexts.
Indicates that the cast may be invoked implicitly in any context.
Cast implementation functions may 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. (Since the grammar presently permits only certain built-in data types to have type modifiers, this feature is of no use for user-defined target types, but we mention it for completeness.)
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.
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.
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: There is one small lie in the preceding paragraph: there is still one case in which pg_cast will be used to resolve the meaning of an apparent function call. If a function call name(x) matches no actual function, but name is the name of a data type and pg_cast shows a binary-compatible cast to this type from the type of x, then the call will be construed as an explicit cast. This exception is made so that binary-compatible casts can be invoked using functional syntax, even though they lack any function.
To create a cast from type text to type int4 using the function int4(text):
CREATE CAST (text AS int4) WITH FUNCTION int4(text);
(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-compatible types or extra arguments to implementation functions. AS IMPLICIT is a PostgreSQL extension, too.