CREATE_TRANSFORM(7) - Man pages

CREATE TRANSFORM(7)      PostgreSQL 15.7 Documentation     CREATE TRANSFORM(7)

NAME
       CREATE_TRANSFORM - define a new transform

SYNOPSIS
       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 [, ...]) ]
       );

DESCRIPTION
       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 be able 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.

PARAMETERS
       type_name
           The name of the data type of the transform.

       lang_name
           The name of the language of the transform.

       from_sql_function_name[(argument_type [, ...])]
           The name of the function for converting the type from the SQL
           environment to the language. It must take one argument of type
           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. (But it is not allowed to declare an SQL-level function
           returning internal without 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.

       to_sql_function_name[(argument_type [, ...])]
           The name of the function for converting the type from the language
           to the SQL environment. It must take one argument of type 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.

NOTES
       Use DROP TRANSFORM to remove transforms.

EXAMPLES
       To create a transform for type hstore and language plpython3u, first
       set up the type and the language:

           CREATE TYPE hstore ...;

           CREATE EXTENSION plpython3u;

       Then create the necessary functions:

           CREATE FUNCTION hstore_to_plpython(val internal) RETURNS internal
           LANGUAGE C STRICT IMMUTABLE
           AS ...;

           CREATE FUNCTION plpython_to_hstore(val internal) RETURNS hstore
           LANGUAGE C STRICT IMMUTABLE
           AS ...;

       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.

       The contrib section contains a number of extensions that provide
       transforms, which can serve as real-world examples.

COMPATIBILITY
       This form of CREATE TRANSFORM is a PostgreSQL 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
       PostgreSQL.

SEE ALSO
       CREATE FUNCTION (CREATE_FUNCTION(7)), CREATE LANGUAGE
       (CREATE_LANGUAGE(7)), CREATE TYPE (CREATE_TYPE(7)), DROP TRANSFORM
       (DROP_TRANSFORM(7))

PostgreSQL 15.7                      2024                  CREATE TRANSFORM(7)

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