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CREATE CAST

CREATE CAST — define a new cast

Synopsis

CREATE CAST (source_type AStarget_type)    WITH FUNCTIONfunction_name [ (argument_type [, ...]) ]    [ AS ASSIGNMENT | AS IMPLICIT ]CREATE CAST (source_type AStarget_type)    WITHOUT FUNCTION    [ AS ASSIGNMENT | AS IMPLICIT ]CREATE CAST (source_type AStarget_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 typefloat8 by invoking a previously specified function, in this casefloat8(int4). (If no suitable cast has been defined, the conversion fails.)

Two types can bebinary 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 typestext andvarchar are binary coercible both ways. Binary coercibility is not necessarily a symmetric relationship. For example, the cast fromxml totext 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 anI/O conversion cast by using theWITH 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 explicitCAST(x AStypename) orx::typename construct.

If the cast is markedAS ASSIGNMENT then it can be invoked implicitly when assigning a value to a column of the target data type. For example, supposing thatfoo.f1 is a column of typetext, then:

INSERT INTO foo (f1) VALUES (42);

will be allowed if the cast from typeinteger to typetext is markedAS ASSIGNMENT, otherwise not. (We generally use the termassignment cast to describe this kind of cast.)

If the cast is markedAS IMPLICIT then it can be invoked implicitly in any context, whether assignment or internally in an expression. (We generally use the termimplicit 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 typeinteger andnumeric respectively. There is nointeger+numeric operator in the system catalogs, but there is anumeric+numeric operator. The query will therefore succeed if a cast frominteger tonumeric is available and is markedAS 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 fromnumeric tointeger. If that cast were markedAS IMPLICIT — which it is not — then the parser would be faced with choosing between the above interpretation and the alternative of casting thenumeric constant tointeger and applying theinteger+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 mixednumeric-and-integer expression asnumeric; 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 causePostgreSQL 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 fromint2 toint4 can reasonably be implicit, but the cast fromfloat8 toint4 should probably be assignment-only. Cross-type-category casts, such astext toint4, are best made explicit-only.

To be able to create a cast, you must own the source or the target data type and haveUSAGE 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

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 typeinteger; it receives the type modifier associated with the destination type, or-1 if there is none. The third argument, if present, must be typeboolean; it receivestrue 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

UseDROP 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, andchar(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 istypename(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. SincePostgreSQL 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.

Examples

To create an assignment cast from typebigint to typeint4 using the functionint4(bigint):

CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;

(This cast is already predefined in the system.)

Compatibility

TheCREATE CAST command conforms to theSQL standard, except that SQL does not make provisions for binary-coercible types or extra arguments to implementation functions.AS IMPLICIT is aPostgreSQL extension, too.

See Also

CREATE FUNCTION,CREATE TYPE,DROP CAST