Postgres adding unexisting varchar array - arrays

Question

I have the following table:
CREATE TABLE fun (
id uuid not null,
tag varchar[] NOT NULL,
CONSTRAINT fun_pkey PRIMARY KEY(id, tag)
);
CREATE UNIQUE INDEX idx_fun_id ON fun USING btree (id);
Then I inserted a data into the table
insert into fun (id, tag)
values('d7f17de9-c1e9-47ba-9e3d-cd1021c644d2', array['123','234'])
So currently, the value of my tag is ["123", "234"]
How can I add the value of the array, and ignore any of the existing varchar, only adding the non-existing one?
currently, this is how I approach it
update fun
set tag = tag || array['234','345']
where id = 'd7f17de9-c1e9-47ba-9e3d-cd1021c644d2'
but my tag will become ["123", "234", "234", "345"]. The value of 234 becomes a duplicated one. What I need to achieve is the value of the tag becomes ["123", "234", "345"]

Answer 1

There is no built-in function to only append unique elements, but it's easy to write one:
create function append_unique(p_one text[], p_two text[])
returns text[]
as
$$
select array(select *
from unnest(p_one)
union
select *
from unnest(p_two));
$$
language sql
immutable;
Then you can use it like this:
update fun
set tag = append_unique(tag,array['234','345'])
where id = 'd7f17de9-c1e9-47ba-9e3d-cd1021c644d2'
Note that this does not preserve the order of the items.
A function that preserves the order of the elements of the existing array and appends the elements of the second one in the order provided would be:
create function append_unique(p_one text[], p_two text[])
returns text[]
as
$$
select p_one||array(select x.item
from unnest(p_two) with ordinality as x(item,idx)
where x.item <> all (p_one)
order by x.idx);
$$
language sql
immutable;