Defines a new transform
CREATE [ OR REPLACE ] TRANSFORM FOR <type_name> LANGUAGE <lang_name> (
FROM SQL WITH FUNCTION <from_sql_function_name> [ (<argument_type> [, ...]) ],
TO SQL WITH FUNCTION <to_sql_function_name> [ (<argument_type> [, ...]) ]
CREATE TRANSFORM defines a new transform.
CREATE OR REPLACE TRANSFORM will either create a new transform, or replace an existing definition.
A transform specifies how to adapt a data type to a procedural language. For example, when writing a function in PL/Python using the
hstore type, PL/Python has no prior knowledge how to present
hstore values in the Python environment. Language implementations usually default to using the text representation, but that is inconvenient when, for example, an associative array or a list would be more appropriate.
A transform specifies two functions:
A "from SQL" function that converts the type from the SQL environment to the language. This function will be invoked on the arguments of a function written in the language.
A "to SQL" function that converts the type from the language to the SQL environment. This function will be invoked on the return value of a function written in the language.
It is not necessary to provide both of these functions. If one is not specified, the language-specific default behavior will be used if necessary. (To prevent a transformation in a certain direction from happening at all, you could also write a transform function that always errors out.)
To create a transform, you must own and have
USAGE privilege on the type, have
USAGE privilege on the language, and own and have
EXECUTE privilege on the from-SQL and to-SQL functions, if specified.
internal and return type
internal. The actual argument will be of the type for the transform, and the function should be coded as if it were. (An SQL-level function returning
internal must declare at least one argument of type
internal.) The actual return value will be something specific to the language implementation. If no argument list is specified, the function name must be unique in its schema.
internal and return the type that is the type for the transform. The actual argument value will be something specific to the language implementation. If no argument list is specified, the function name must be unique in its schema.
Use DROP TRANSFORM to remove transforms.
To create a transform for type
hstore and language
plpython3u, first set up the type and the language:
CREATE EXTENSION IF NOT EXISTS hstore;
CREATE EXTENSION plpython3u;
Then create the necessary functions:
CREATE FUNCTION hstore_to_plpython(val internal) RETURNS internal
LANGUAGE C STRICT IMMUTABLE
CREATE FUNCTION plpython_to_hstore(val internal) RETURNS hstore
LANGUAGE C STRICT IMMUTABLE
And finally create the transform to connect them all together:
CREATE TRANSFORM FOR hstore LANGUAGE plpython3u (
FROM SQL WITH FUNCTION hstore_to_plpython(internal),
TO SQL WITH FUNCTION plpython_to_hstore(internal)
In practice, these commands would be wrapped up in an extension.
This form of
CREATE TRANSFORM is a Greenplum Database extension. There is a
CREATE TRANSFORM command in the SQL standard, but it is for adapting data types to client languages. That usage is not supported by Greenplum.
Parent topic: SQL Commands