CREATE TRANSFORM — define a new transform
CREATE [ OR REPLACE ] TRANSFORM FORtype_name
LANGUAGElang_name
( FROM SQL WITH FUNCTIONfrom_sql_function_name
[ (argument_type
[, ...]) ], TO SQL WITH FUNCTIONto_sql_function_name
[ (argument_type
[, ...]) ] );
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:
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.
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.
If no argument list is specified, the function name must be unique in
its schema.
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.
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 hstore
and language
plpythonu
, first set up the type and the language:
CREATE TYPE hstore ...; CREATE EXTENSION 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 an extension.
The contrib
section contains a number of extensions
that provide transforms, which can serve as real-world examples.
This form of CREATE TRANSFORM
is a
LightDB 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 LightDB.
CREATE FUNCTION, CREATE LANGUAGE, CREATE TYPE, DROP TRANSFORM