What is TRANSFORM FOR TYPE?

Question

PostgreSQL 11 introduces TRANSFORM FOR TYPE, from these docs by HP, PostgreSQL 11 New_Features

postgres=# CREATE EXTENSION jsonb_plperl CASCADE ;
NOTICE: installing required extension "plperl"
CREATE EXTENSION

postgres=> CREATE OR REPLACE FUNCTION fperl(val jsonb)
RETURNS jsonb
TRANSFORM FOR TYPE jsonb
LANGUAGE plperl
AS $$
 return $_[0] ;
$$ ;
CREATE FUNCTION

postgres=> SELECT fperl('{"1":1,"example": null}'::jsonb) ;
 fperl
---------------------------
{"1": 1, "example": null}
(1 row)

But it seems this would work the very same way if I remove TRANSFORM FOR TYPE jsonb

Answer 1

That's a totally useless example. I believe what TRANSFORM FOR TYPE is supposed to support is something like this,

postgres=> CREATE OR REPLACE FUNCTION fperl(val jsonb)
RETURNS jsonb
TRANSFORM FOR TYPE jsonb
LANGUAGE plperl
AS $$
 return $_[0]->{foo};
$$ ;

SELECT fperl('{"foo":"bar"}'::jsonb) ;

You can see in the above that we actually see the jsonb object as a perl hash and can access the members.

jsonb_plperl is the new stuff here. CREATE TRANSFORM dates to PostgreSQL 9.5.