## CREATE TRANSFORM

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.

`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.

## Notes

Use DROP TRANSFORM to remove transforms.

## Examples

To create a transform for type `hstore` and language `plpythonu`, first set up the type and the language:

    CREATE TYPE hstore ...;

    CREATE LANGUAGE plpythonu ...;

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 plpythonu (
        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 extensions.

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 LANGUAGE, CREATE TYPE, DROP TRANSFORM