CREATE TRANSFORM — define a new transform
CREATE [ OR REPLACE ] TRANSFORM FOR
lang_name( FROM SQL WITH FUNCTION
argument_type[, ...]) ], TO SQL WITH FUNCTION
argument_type[, ...]) ] );
CREATE TRANSFORM defines a new
CREATE OR REPLACE TRANSFORM
will either create a new transform, or replace an existing
A transform specifies how to adapt a data type to a procedural
language. For example, when writing a function in PL/Python using
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
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.
The name of the data type of the transform.
The name of the language of the transform.
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
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
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.
Use DROP TRANSFORM to remove transforms.
To create a transform for type
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.
contrib section contains a
number of extensions that provide transforms, which can serve as
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.
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