CREATE CAST

CREATE CAST

7

SQL - Language Statements

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 \[ AS ASSIGNMENT \| AS IMPLICIT \]

Description {#sql-createcast-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 float8);

converts the integer constant 42 to type `float8` by invoking a
previously specified function, in this case `float8(int4)`. (If no
suitable cast has been defined, the conversion fails.)

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 types `text` and `varchar` are binary coercible both ways.
Binary coercibility is not necessarily a symmetric relationship. For
example, the cast from `xml` to `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 binary compatible.)

You can define a cast as an I/O conversion cast by using the
`WITH INOUT` syntax. An I/O conversion cast is performed by invoking the
output function of the source data type, and passing the resulting
string to the input function of the target data type. In many common
cases, this feature avoids the need to write a separate cast function
for conversion. An I/O conversion cast acts the same as a regular
function-based cast; only the implementation is different.

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 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. (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` MDASH 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` MDASH which it is not MDASH 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` `+` `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
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.

> **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
> [???](#sql-createtype) for more information.

To be able to create a cast, you must own the source or the target data
type and have `USAGE` privilege on the other 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.)

Parameters
==========

source\_type

:   The name of the source data type of the cast.

target\_type

:   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. If
    no argument list is specified, the function name must be unique in
    its schema.

`WITHOUT FUNCTION`

:   Indicates that the source type is binary-coercible to the target
    type, so no function is required to perform the cast.

`WITH INOUT`

:   Indicates that the cast is an I/O conversion cast, performed by
    invoking the output function of the source data type, and passing
    the resulting string to the input function of the target data type.

`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 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 `-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 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 supports 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 cast steps, one to convert between data types and a second
to apply the modifier.

A cast to or from a domain type currently has no effect. Casting to or
from a domain uses the casts associated with its underlying type.

Notes {#sql-createcast-notes}
=====

Use [`DROP CAST`](#sql-dropcast) 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.

cast

I/O conversion

It is normally not necessary to create casts between user-defined types
and the standard string types (`text`, `varchar`, and `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.)

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\_cast provides 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\_cast entry 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.

> **Note**
>
> There is also an exception to the exception: I/O conversion casts from
> composite types to string types cannot be invoked using functional
> syntax, but must be written in explicit cast syntax (either `CAST` or
> `::` notation). This exception was added because after the
> introduction of automatically-provided I/O conversion casts, it was
> found too easy to accidentally invoke such a cast when a function or
> column reference was intended.

Examples {#sql-createcast-examples}
========

To create an assignment cast from type `bigint` to type `int4` using the
function `int4(bigint)`:

    CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;

(This cast is already predefined in the system.)

Compatibility {#sql-createcast-compat}
=============

The `CREATE CAST` command conforms to the SQL standard, except that SQL
does not make provisions for binary-coercible types or extra arguments
to implementation functions. `AS IMPLICIT` is a PostgreSQL extension,
too.

See Also {#sql-createcast-seealso}
========

[???](#sql-createfunction), [???](#sql-createtype), [???](#sql-dropcast)
