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 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
arguments,
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. Cast functions must be marked immutable or stable.
- 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 catalogs. (The built-in
cast functions
still follow this naming scheme, but they have to be shown as
casts in pg_cast now.)
