Postgres - How to cast an array of enum_1 to enum_2? - sql

Question

I'm trying to modify the values of an enum in my schema ("feature" in the below example).
I'm trying to do this by renaming the old enum and introducing a new one that has the values I want, and then altering the table definition to the new enum.
I'm following this blog post here: https://blog.yo1.dog/updating-enum-values-in-postgresql-the-safe-and-easy-way/.
But instead of the column being a simple column of the enum, my column is actually an array of the enum.
When I try to run the alter table statement in the below statements, I get the error:
[42804] ERROR: column "features" is of type feature_old[] but expression is of type feature_v2[] Hint: You will need to rewrite or cast the expression.
alter type feature rename to feature_old;
create type feature_v2 as enum (
'enable_create_keyword',
'enable_make_payment',
'enable_test_data_flags'
);
-- ... cleanup of column array values to be compatible with new enum ...
alter table app_user alter column features type feature_v2
using features::feature_old[]::feature_v2[];
drop type feature_old;
But, I'm lost - what should the cast expression look like?
Postgres version is 9.6
EDIT
This is the relevant part of the previous version's schema DDL for the feature enum and app_user table that was requested by #VaoTsun.
-- feature enum and column
create type feature as enum ('enable_create_keyword', 'enable_make_payment');
comment on type feature is
'if default functionality is disabled feature name starts with enable_, if default is enabled starts with disable_'
;
alter table app_user add column
features feature[] not null default ARRAY[]::feature[];
-- feature data
update app_user
set features = ARRAY['enable_create_keyword', 'enable_make_payment']::feature[]
where email = 'test1#example.com';
update app_user
set features = ARRAY['enable_create_keyword']::feature[]
where email = 'test2#example.com';

Answer 1

Thanks to both Vao Tsun and Nick Barnes; this is the code that appears to work for me. I have marked Vao Tsun's answer as correct. Any answers that provide a more concise version would be gratefully upvoted.
alter type feature rename to feature_old;
create type feature_v2 as enum (
'enable_create_keyword',
'enable_make_payment',
'enable_test_data_flags'
);
alter table app_user alter column features drop default ;
alter table app_user alter column features type feature_v2[]
using features::feature_old[]::text[]::feature_v2[];
alter table app_user alter column features set default ARRAY[]::feature_v2[];
drop type feature_old;

Answer 2

with assumption that old enum has same values, but less, you should be able to simply cast it's value to text and then to a v2:
try this:
t=# create or replace function feature2v2(feature_old) returns feature_v2 as
$$
select $1::text::feature_v2;
$$
language sql strict;
CREATE FUNCTION
t=# create cast (feature_old AS feature_v2) WITH FUNCTION feature2v2(feature_old) AS ASSIGNMENT;
CREATE CAST
gives me:
t=# alter table app_user alter column features type feature_v2[]
using features::feature_v2[];
ALTER TABLE
t=# \d+ app_user
Table "postgres.app_user"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
----------+--------------+-----------+----------+------------------------+----------+--------------+-------------
email | text | | | | extended | |
features | feature_v2[] | | not null | ARRAY[]::feature_old[] | extended | |
t=# \dT+ feature_v2
List of data types
Schema | Name | Internal name | Size | Elements | Owner | Access privileges | Description
----------+------------+---------------+------+------------------------+----------+-------------------+-------------
postgres | feature_v2 | feature_v2 | 4 | enable_create_keyword +| postgres | |
| | | | enable_make_payment +| | |
| | | | enable_test_data_flags | | |
(1 row)
which looks like what you expect
UPDATE
catching up with Nick Barnes comments - creating a cast here is overhead and leaves bad defualt for column, the right approach her is:
alter table app_user alter column features drop default;
alter table app_user alter column features type feature_v2[] using features::feature_old[]::text[]::feature_v2[];
alter table app_user alter column features set default ARRAY[]::feature_v2[];
Leaving the previous version untouched to demonstrate the bad approach with hints how it is bad