CREATE TRANSFORM(7) | PostgreSQL 16.3 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:
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:
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))
2024 | PostgreSQL 16.3 |