Getting list of all children in “adjacency list” tree structure

So, you have a table which looks like this:

# \d test
                           Table "public.test"
  Column   |  Type   |                     Modifiers
 id        | integer | not null default nextval('test_id_seq'::regclass)
 parent_id | integer |
 x         | text    |
    "test_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "test_parent_id_fkey" FOREIGN KEY (parent_id) REFERENCES test(id)
Referenced by:
  "test_parent_id_fkey" IN test FOREIGN KEY (parent_id) REFERENCES test(id)

And you would like to easily get all children starting from given node?

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