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CREATE CAST(7) SQL Commands CREATE CAST(7)
NAME
CREATE CAST - define a new cast
SYNOPSIS
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 (sourcetype AS targettype)
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 func-
tion 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 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 result to the input function of the target data
type.
By default, a cast can be invoked only by an explicit cast request, that is an explicit CAST(x AS typename) or x::type-
name 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 knowl-
edge about that.
It is wise to be conservative about marking casts as implicit. An overabundance of implicit casting paths can cause Post-
greSQL 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-pre-
serving transformations between types in the same general type category. For example, the cast from int2 to int4 can rea-
sonably 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. 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
sourcetype
The name of the source data type of the cast.
targettype
The name of the target data type of the cast.
funcname(argtypes)
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.
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 result 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 modi-
fier 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 behav-
iors 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 iden-
tical source and target types if it has a cast implementation function with more than one argument. This is used to rep-
resent 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 represents con-
verting from one type to another and applying a length coercion in a single step. When no such entry is available, coer-
cion to a type that uses a type modifier involves two steps, one to convert between data types and a second to apply the
modifier.
NOTES
Use DROP CAST [drop_cast(7)] 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.)
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 intro-
duction of schemas and to be able to represent binary-coercible 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 func-
tions 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 con-
struct 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 func-
tional syntax.
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.
SEE ALSO
CREATE FUNCTION [create_function(7)], CREATE TYPE [create_type(7)], DROP CAST [drop_cast(7)]
SQL - Language Statements 2011-09-22 CREATE CAST(7)
