Alter domain type / avoiding table rewrite - Mailing list pgsql-general

So I have a situation where I would like to modify a field that is currently a domain type over a varchar(9)

Specifically:

CREATE DOMAIN old_type AS varchar(9)

This isn't ideal, let's just say.. legacy.

I wish to modify this type.. ideally to a text type with a length constraint.. or even just a slightly larger varchar(12) would suffice.. 

CREATE DOMAIN new_type AS text;

ALTER DOMAIN new_type ADD CONSTRAINT check_len CHECK ((length(VALUE) <= 12)) NOT VALID;

ALTER TABLE target ALTER

COLUMN value SET DATA TYPE new_type;

But it seems impossible to achieve either without a full table rewrite.

This seems to boil down to DOMAIN types not being considered as binary compatible..

I've tried using a custom CAST..

CREATE CAST (old_type AS new_type) WITHOUT FUNCTION AS IMPLICIT;

But that blows up, with:

WARNING:  cast will be ignored because the source date is a domain

ERROR: domain data types must not Be marked binary compatible

So I'm a little stuck at this point.

I feel like - if I can prove that the binary representation of both domains are truly identical - I might be forced to modify the system tables as a work around.. that scares me on a production system.

Is there a way around this that i'm not seeing?

I'm on PostgreSQL 9.6.2