CREATE CAST
Defines a new 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]
CREATE CAST (<sourcetype> AS <targettype>)
WITH INOUT
[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 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 may be binary coercible, which means that the types can be converted into one another without invoking any function. This requires that corresponding values use the same internal representation. For instance, the types text
and varchar
are binary coercible in both directions. 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. The term assignment cast is typically used 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. The term implicit cast is typically used 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. This query succeeds if a cast from integer
to numeric
exists (it does) and is marked AS IMPLICIT
, which in fact it is. The parser applies only the implicit cast and resolves the query as if it had been written as the following:
SELECT CAST ( 2 AS numeric ) + 4.0;
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, the parser 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
; the parser has 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 Apache Cloudberry to choose surprising interpretations of commands, or to be unable to resolve commands at all because there are multiple possible interpretations. A good general rule 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 described above. The parser uses a fallback heuristic based on type categories and preferred types that helps 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.)