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CREATE TRANSFORM(7)	PostgreSQL 9.6.3 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:

       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 9.6.3		     2017		   CREATE TRANSFORM(7)

NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | NOTES | EXAMPLES | COMPATIBILITY | SEE ALSO

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