create_transform(7)

—

NAME

CREATE_TRANSFORM - define a new transform

SYNOPSIS

bash

CREATE [ OR REPLACE ] TRANSFORM FOR \ntype_name\n LANGUAGE \nlang_name\n (
\n
    FROM SQL WITH FUNCTION \nfrom_sql_function_name\n [ (\nargument_type\n [, ...]) ],
\n
    TO SQL WITH FUNCTION \nto_sql_function_name\n [ (\nargument_type\n [, ...]) ]
);

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:

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

lang_name

from_sql_function_name[(argument_type [, ...])]

to_sql_function_name[(argument_type [, ...])]

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:

bash

CREATE TYPE hstore ...;
CREATE EXTENSION plpython3u;

Then create the necessary functions:

bash

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:

bash

CREATE TRANSFORM FOR hstore LANGUAGE plpython3u (
\n
    FROM SQL WITH FUNCTION hstore_to_plpython(internal),
\n
    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.