PostgreSQL constraint trigger to check if each array element is unique within the table

trg_check_array_elements_column_uniq.sql

-------------------------------------------------------------------------------
-- EXAMPLE USAGE
--
-- create table specialization (
--   id bigserial,
--   name text not null,
--   study_plan_codes text[] not null default '{}'
-- )
--
-- create constraint trigger study_plan_codes_unique_check
--   after insert or update of study_plan_codes on specialization
--   for each row
--   when (cardinality(NEW.study_plan_codes) > 0)
--   execute function trg_check_array_elements_column_uniq('study_plan_codes');
-------------------------------------------------------------------------------

create function trg_check_array_elements_column_uniq ()
returns trigger
language plpgsql
as $$
  declare
    col_name text := TG_ARGV[0];
    duplicates text;
    ok boolean;
  begin
    if TG_NARGS != 1 then
      raise 'invalid number of arguments: expected 1, but given %', TG_NARGS using
        detail = 'The function was called from trigger "' || TG_NAME || '".';
    end if;

    execute format('
      select not exists (
        select 1
        from %2$I
        where %1$I && $1.%1$I
        offset 1
      )',
      col_name, TG_TABLE_NAME)
    into strict ok
    using NEW;

    if ok then
      return null;
    else
      execute format('
        select string_agg(quote_literal(value), '', '')
        from (
          select unnest(%1$I) as value
          from %2$I
          where %1$I && $1.%1$I
          group by value
          having count(*) > 1
        ) x',
        col_name, TG_TABLE_NAME)
      into strict duplicates
      using NEW;

      raise 'duplicate array element in column "%"', col_name using
        detail = 'Following element(s) already exist in table "' || TG_TABLE_NAME || '": ' || duplicates || '.',
        column = col_name,
        table = TG_TABLE_NAME,
        errcode = 'unique_violation';
    end if;
  end;
$$;
comment on function trg_check_array_elements_column_uniq is
  'A trigger function for AFTER INSERT OR UPDATE trigger that checks if all elements '
  'of the specified array column are unique across the table. If duplicate elements are '
  'are found, an exception with ERRCODE "unique_violation" is raised.'
  'This function does not check for duplicates inside an array (in a row), just between '
  'the arrays in the column.'
  'The column name to check must be passed as the first and only argument of the function.';