CREATE CAST — define a new cast
CREATE CAST (source_typeAStarget_type) WITH FUNCTIONfunction_name[ (argument_type[, ...]) ] [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_typeAStarget_type) WITHOUT FUNCTION [ AS ASSIGNMENT | AS IMPLICIT ] CREATE CAST (source_typeAStarget_type) WITH INOUT [ AS ASSIGNMENT | AS IMPLICIT ]
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( or
x AS
typename)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 —
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 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
LightDB 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.
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 CREATE TYPE 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.)
source_typeThe name of the source data type of the cast.
target_typeThe 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 FUNCTIONIndicates that the source type is binary-coercible to the target type, so no function is required to perform the cast.
WITH INOUTIndicates 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 ASSIGNMENTIndicates that the cast can be invoked implicitly in assignment contexts.
AS IMPLICITIndicates 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.
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.
It is normally not necessary to create casts between user-defined types
and the standard string types (text, varchar, and
char(, as well as user-defined types that
are defined to be in the string category). LightDB
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.)
n)
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 LightDB 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.
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.
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.
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.)
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 LightDB
extension, too.