/usr/man2/cat.l/create_cast.l.Z
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 ]
DESCRIPTION
CREATE CAST defines a new cast. A cast specifies how to perform a con-
version 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 con-
verted into one another ``for free'' without invoking any function.
This requires that corresponding values use the same internal represen-
tation. 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) or x::typename 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 exam-
ple, 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 explic-
itly, 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 over-
abundance of implicit casting paths can cause PostgreSQL to choose sur-
prising interpretations of commands, or to be unable to resolve com-
mands 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 gen-
eral type category. For example, the cast from int2 to int4 can reason-
ably 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 conver-
sion 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 may 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 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 con-
text.
Cast implementation functions may have one to three arguments. The
first argument type must be identical to 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 -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
spec 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.)
Ordinarily a cast must have different source and target data types.
However, it is allowed to declare a cast with identical source and tar-
get types if it has a cast implementation function with more than one
argument. This is used to represent type-specific length coercion func-
tions 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 argu-
ment. (Since the grammar presently permits only certain built-in data
types to have type modifiers, this feature is of no use for user-
defined target types, but we mention it for completeness.)
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.
NOTES
Use DROP CAST [drop_cast(l)] 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 differ-
ent type was automatically a cast function. This convention has been
abandoned in face of the introduction of schemas and to be able to rep-
resent 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 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 multi-
ple conversion functions from different types that all use the target
type's name.
Note: There is one small lie in the preceding paragraph: there
is still one case in which pg_cast will be used to resolve the
meaning of an apparent function call. If a function call name(x)
matches no actual function, but name is the name of a data type
and pg_cast shows a binary-compatible cast to this type from the
type of x, then the call will be construed as an explicit cast.
This exception is made so that binary-compatible casts can be
invoked using functional syntax, even though they lack any func-
tion.
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 the SQL standard, except that SQL
does not make provisions for binary-compatible types or extra arguments
to implementation functions. AS IMPLICIT is a PostgreSQL extension,
too.
SEE ALSO
CREATE FUNCTION [create_function(l)], CREATE TYPE [create_type(l)],
DROP CAST [drop_cast(l)]
SQL - Language Statements 2005-11-05 CREATE CAST()
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