CREATE CAST(l)SQL - Language Statements (2002-11-22CREATE CAST(l)
NAME
CREATE CAST - define a user-defined cast
SYNOPSIS
CREATE CAST (sourcetype AS targettype)
WITH FUNCTION funcname (argtype)
[ 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 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
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CREATE CAST(l)SQL - Language Statements (2002-11-22CREATE CAST(l)
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
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CREATE CAST(l)SQL - Language Statements (2002-11-22CREATE CAST(l)
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.)
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 SQL99, except that SQL99
does not make provisions for binary compatible types. AS
IMPLICIT is a PostgreSQL extension, too.
SEE ALSO
CREATE FUNCTION [create_function(l)], CREATE TYPE
[create_type(l)], DROP CAST [drop_cast(l)], PostgreSQL
Programmer's Guide
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