Algebraic Data Types in Postgres - sql

Question

Is it possible to create an Algebraic Data Type in Postgres and then use it as a column type?
For example:
CREATE TYPE hoofed AS ENUM('horse', 'goat');
CREATE TYPE monkey AS ENUM('chimp','macaque');
CREATE TYPE ANIMAL AS ENUM(hoofed, monkey);
This fails with:
syntax error at or near "hoofed"
LINE 1: CREATE TYPE ANIMAL AS ENUM(hoofed, monkey);
Is it possible to do something like this?
Ultimately what I would then like to be able to do is something like so:
CREATE TABLE zoo (
a ANIMAL,
name text
);
INSERT INTO zoo(a, name) VALUES('horse', 'bob');
INSERT INTO zoo(a, name) VALUES('macaque', 'jimmy');
And for both of the records to be independently valid.
EDIT: #Abihabi87's response below does allow me to create, in effect, a product type, but it still does not allow me to create a union type as desired.

Answer 1

You cant create type enum from others enum type:
you can create ANIMAL that like:
CREATE TYPE ANIMAL AS (h hoofed,m monkey);
Example in use:
CREATE TABLE your_table
(
a ANIMAL
);
INSERT INTO your_table(a) select (select ('horse','macaque')::ANIMAL);

Answer 2

With ENUM types, you cannot achieve dynamic type composition/union. However, with DOMAIN types, you could achieve something similar:
create function valid_any_domain(anyelement, variadic regtype[])
returns boolean
language plpgsql
immutable
as $func$
declare
t regtype;
begin
foreach t in array $2 loop
begin
execute format('select $1::%s', t) using $1;
exception
when not_null_violation or check_violation then
continue;
end;
return true;
end loop;
return false;
end;
$func$;
create domain hoofed as text
check (value in ('horse', 'goat'));
create domain monkey as text
check (value in ('chimp','macaque'));
create domain animal as text
check (valid_any_domain(value, 'hoofed', 'monkey'));
Changing the base types will dynamically change the composite/union type too, but still requires a manual constraint validation (especially, when some value(s) are removed from the valid spectrum):
alter domain hoofed drop constraint hoofed_check;
alter domain hoofed add check (value in ('horse', 'goat', 'zebra'));
alter domain animal validate constraint animal_check;
http://rextester.com/MBVC62095
Note: however, with DOMAIN types, you will lose an ENUM property: the custom ordering. DOMAINs will always use the underlying type's ordering.

Answer 3

Use the function:
create or replace function create_enum(name, variadic regtype[])
returns void language plpgsql as $$
begin
execute format(
'create type %I as enum(%s)',
$1,
string_agg(quote_literal(enumlabel), ',' order by enumtypid, enumsortorder))
from pg_enum
where enumtypid = any($2);
end $$;
Pass the name of a new type and a list of enum types as arguments:
select create_enum('animal', 'hoofed', 'monkey');
select enum_range(null::animal) as animal;
animal
----------------------------
{horse,goat,chimp,macaque}
(1 row)

Answer 4

Effectively you are trying to merge two enum types.
There are some open questions:
Can there be duplicate strings?
Is the design supposed to be static (changes to enum type hoofed do not change type animal later) or dynamic (the opposite).
Merge exactly two enum types or more?
Since the order of elements is significant, what is the order of elements in animal supposed to be?
Is this a one-time operation or intended for repeated use?
Assuming no duplicates, static design, two enum types, existing order of elements as appended and one-time operation.
You can use the built-in enum support function enum_range(anyenum) to get an array of all elements for a given enum type.
DO
$$
BEGIN
EXECUTE (
SELECT 'CREATE TYPE animal AS ENUM ('
|| array_to_string(enum_range(null::hoofed)::text[]
|| enum_range(null::monkey)::text[], ''',''')
|| ''')'
);
END
$$;