PostgreSQL 7.4beta4 Documentation
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# CREATE CAST

## Name

CREATE CAST -- define a new cast

## Synopsis

    CREATE CAST (sourcetype AS targettype)
        WITH FUNCTION funcname (argtype)
        [ AS ASSIGNMENT | AS IMPLICIT ]

    CREATE CAST (sourcetype AS targettype)
        WITHOUT FUNCTION
        [ 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 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 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.)

## Parameters

sourcetype

  The name of the source data type of the cast.

targettype

  The name of the target data type of the cast.

funcname(argtype)

  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.

WITHOUT FUNCTION

  Indicates that the source type and the target type are binary compatible, so no function is required to perform the cast.

AS ASSIGNMENT

  Indicates that the cast may be invoked implicitly in assignment contexts.

AS IMPLICIT

  Indicates that the cast may be invoked implicitly in any context.

## 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.

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 now.)

## Examples

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.)

## Compatibility

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.

## See Also

CREATE FUNCTION, CREATE TYPE, DROP CAST

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