There are some moments I should mention:
1. {"1":1}::jsonb is transformed into HV {"1"=>"1"}, while
["1","2"]::jsonb is transformed into AV ["1", "2"]
2. If there is a numeric value appear in jsonb, it will be transformed
to SVnv through string (Numeric->String->SV->SVnv). Not the best
solution, but as far as I understand this is usual practise in
postgresql to serialize Numerics and de-serialize them.
3. SVnv is transformed into jsonb through string
(SVnv->String->Numeric).
An example may also be helpful to understand extension. So, as an
example, function "test" transforms incoming jsonb into perl,
transforms it back into jsonb and returns it.
create extension jsonb_plperl cascade;
create or replace function test(val jsonb)
returns jsonb
transform for type jsonb
language plperl
as $$
return $_[0];
$$;
select test('{"1":1,"example": null}'::jsonb);
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