CREATE CAST (sourcetype AS targettype) WITH FUNCTION funcname (argtype) [ 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), x::typename, or typename(x) construct.
If the cast is marked AS ASSIGNMENT then it can be invoked implicitly when assigning 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 arguments,
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 path. The argument type must be identical to the source type, the result data type must match the target type of the cast. Cast functions must be marked immutable or stable.
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.
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 catalogs. (The built-in cast functions still follow this naming scheme, but they have to be shown as casts in pg_cast now.)
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 SQL99, except that SQL99 does not make provisions for binary compatible types. AS IMPLICIT is a PostgreSQL extension, too.