distinct column -> For each row returned, return only the unique members of a set.
Think of it as for each row in a projection, concatenate all the column values and return only the strings that are unique.
test_db=# SELECT DISTINCT parent_id, child_id, id FROM test.foo_table ORDER BY parent_id, child_id, id LIMIT 10;
parent_id | child_id | id
-----------+------------+-----------------------------
1000040 | 103 | 1000040|2645405726|0001|103