Defines a new cast.
CREATE CAST (sourcetype AS targettype) WITH FUNCTION funcname (argtypes) [AS ASSIGNMENT | AS IMPLICIT] CREATE CAST (sourcetype AS targettype) WITHOUT FUNCTION [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 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 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) 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, since || takes text operands,
SELECT 'The time is ' || now();
will be allowed only if the cast from type timestamp to text is marked AS IMPLICIT. Otherwise, it will be necessary to write the cast explicitly, for example
SELECT 'The time is ' || CAST(now() AS text);
It is wise to be conservative about marking casts as implicit. An overabundance of implicit casting paths can cause Greenplum Database 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.
- The name of the source data type of the cast.
- The name of the target data type of the cast.
- 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 schema search path. The function's result data type must match the target type of the cast.
- Cast implementation functions may 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. The SQL specification 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 this way.
- 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. (Since the grammar presently permits only certain built-in data types to have type modifiers, this feature is of no use for user-defined target types.)
- 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.
- 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 any context.
Note that in this release of Greenplum Database, user-defined functions used in a user-defined cast must be defined as IMMUTABLE. Any compiled code (shared library files) for custom functions must be placed in the same location on every host in your Greenplum Database array (master and all segments). This location must also be in the LD_LIBRARY_PATH so that the server can locate the files.
Remember that if you want to be able to convert types both ways you need to declare casts both ways explicitly.
It is recommended that you follow the convention of naming cast implementation functions after the target data type, as the built-in cast implementation functions are named. Many users are used to being able to cast data types using a function-style notation, that is typename(x).
To create a cast from type text to type int4 using the function int4(text) (This cast is already predefined in the system.):
CREATE CAST (text AS int4) WITH FUNCTION int4(text);
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 Greenplum Database extension, too.