CREATE TRANSFORM(7) PostgreSQL 9.5.10 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:
o 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.
o 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 (DROP_TRANSFORM(7)) 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_FUNCTION(7)), CREATE LANGUAGE
(CREATE_LANGUAGE(7)), CREATE TYPE (CREATE_TYPE(7)), DROP TRANSFORM
(DROP_TRANSFORM(7))
PostgreSQL 9.5.10 2017 CREATE TRANSFORM(7)
PostgreSQL 9.5.10 - Generated Fri Dec 1 06:10:24 CST 2017