CREATE TRANSFORM

CREATE TRANSFORM

7

SQL - Language Statements

CREATE TRANSFORM

define a new transform

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 {#sql-createtransform-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:

-   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.

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. 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.

Notes {#sql-createtransform-notes}
=====

Use [`DROP TRANSFORM`](#sql-droptransform) to remove transforms.

Examples {#sql-createtransform-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 {#sql-createtransform-compat}
=============

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 {#sql-createtransform-seealso}
========

[???](#sql-createfunction), [???](#sql-createlanguage),
[???](#sql-createtype), [???](#sql-droptransform)
