|The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference
by The PostgreSQL Global Development Group
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1.41 CREATE CAST
CREATE CAST -- define a new cast
CREATE CAST (source_type AS target_type) WITH FUNCTION function_name (argument_type [, ...]) [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_type AS target_type) WITHOUT FUNCTION [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_type AS target_type) WITH INOUT [ 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 float8);
converts the integer constant 42 to type
invoking a previously specified function, in this case
float8(int4). (If no suitable cast has been defined, the
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
varchar are binary
coercible both ways. Binary coercibility is not necessarily a
symmetric relationship. For example, the cast
text 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
You can define a cast as an I/O conversion cast using
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
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
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
numeric respectively. There is no
numeric operator in the system catalogs,
but there is a
The query will therefore succeed if a cast from
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
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 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
integer expression as
numeric; 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 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
int4 can reasonably be implicit, but the cast from
int4 should probably be
assignment-only. Cross-type-category casts, such as
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 TYPEfor 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.)
- The name of the source data type of the cast.
- The name of the target data type of the cast.
- function_name(argument_type [, ...])
- 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.
- Indicates that the source type is binary-coercible to the target type, so no function is required to perform the cast.
- 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.
- Indicates that the cast can be invoked implicitly in assignment contexts.
- 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
modifier associated with the destination type, or
if there is none. The third argument,
if present, must be type
boolean; it receives
if the cast is an explicit cast,
(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 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.
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.
It is normally not necessary to create casts between user-defined types
and the standard string types (
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 introduction 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 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: Actually the preceding paragraph is an oversimplification: there are two cases in which a function-call construct 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_castprovides 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_castentry 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 functional syntax.
To create an assignment cast from type
bigint to type
int4 using the function
CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
(This cast is already predefined in the system.)
CREATE CAST command conforms to the
except that SQL does not make provisions for binary-coercible
types or extra arguments to implementation functions.
AS IMPLICIT is a PostgreSQL
|ISBN 9781906966058||The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference||See the print edition|