CREATE CAST() SQL Commands CREATE CAST()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 float8);
converts the integer constant 42 to type float8 by invoking a previ‐
ously specified function, in this case float8(int4). (If no suitable
cast has been defined, the conversion fails.)
Two types can 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. (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 alter‐
native 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 knowledge about that.
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 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 and the target type are binary
compatible, so no function is required to perform the cast.
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 con‐
text.
Cast implementation functions can 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.
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(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)). PostgreSQL
will automatically handle a cast to a string type by invoking the other
type's output function, or conversely handle a cast from a string type
by invoking the other type's input function. These automatically-pro‐
vided casts are known as I/O conversion casts. I/O conversion casts to
string types are treated as assignment casts, while I/O conversion
casts from string types are explicit-only. You can override this behav‐
ior by declaring your own cast to replace an I/O conversion 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 suffi‐
ciently surprising that you should think twice about whether it's a
good idea. (A small number of the built-in types do indeed have differ‐
ent 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 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: 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_cast provides a binary-compatible cast to this type from the
type of x, then the call will be construed as a binary-compati‐
ble cast. This exception is made so that binary-compatible 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 con‐
strued as an I/O conversion cast. This exception allows I/O con‐
version casts to be invoked using functional syntax.
EXAMPLES
To create a cast from type bigint to type int4 using the function
int4(bigint):
CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint);
(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(7)], CREATE TYPE [create_type(7)],
DROP CAST [drop_cast(7)]
SQL - Language Statements 2013-02-04 CREATE CAST()
