Subquery select values and save into enum in postgresql - postgresql

Question

I'm trying to remove a value from an existing postgresql enum datatype. My research tells me that I need to remake the enum data type excluding the value I do not want. My idea for accomplishing this is to get all values from the existing data type via
select
e.enumlabel as enum_value
from pg_type t
join pg_enum e on t.oid = e.enumtypid
join pg_catalog.pg_namespace n ON n.oid = t.typnamespace
where t.typname = 'alert_level' where e.enumlabel is not 'value i want to exclude'
create type alert_type2 as enum ('ABOVE QUERY HERE') ::alert_type
and take these values I want and somehow insert them into a new data type
So my question: Is it possible to get values from a subquery and create an enum data type from the query results? Is there a better way to do this?
I've also tried something like this but I cant figure out how to exclude values from the select enum_range.
create type alert_type2 as enum (select enum_range(NULL::alert_type)::text)

Answer 1

You have to exceute a dynamic query inside a function or DO block, example:
create type fruits as enum ('apple', 'pear', 'banana');
do $$
begin
execute format(
'create type fruits_without_pear as enum (%s)',
(
select array_to_string(array_agg(quote_literal(e)), ',')
from unnest(enum_range(null::fruits)) e
where e <> 'pear'
)
);
end $$;
select enum_range(null::fruits_without_pear);
enum_range
----------------
{apple,banana}
(1 row)
Read about:
Executing Dynamic Commands
DO command with example
format() and quote_literal() functions
array_agg()
unnest() and array_to_string().
Notes about used query.
select enum_range(null::fruits)
enum_range
---------------------
{apple,pear,banana}
(1 row)
We want to transform the result to the string 'apple','banana' to use it as a parameter in format().
First, remove 'pear' from the list:
select e
from unnest(enum_range(null::fruits)) e
where e <> 'pear';
e
--------
apple
banana
(2 rows)
next, quote the elements and aggregate them into an array:
select array_agg(quote_literal(e))
from unnest(enum_range(null::fruits)) e
where e <> 'pear'
array_agg
--------------------
{'apple','banana'}
(1 row)
finally, transform an array to string:
select array_to_string(array_agg(quote_literal(e)), ',')
from unnest(enum_range(null::fruits)) e
where e <> 'pear'
array_to_string
------------------
'apple','banana'
(1 row)
However, we should have done it a little simpler ;)
select string_agg(quote_literal(e), ',')
from unnest(enum_range(null::fruits)) e
where e <> 'pear'
array_to_string
------------------
'apple','banana'
(1 row)