CREATE TABLE x (id INT) ;

  INSERT INTO x VALUES (1);
  INSERT INTO x VALUES (2);
  INSERT INTO x VALUES (3);
  INSERT INTO x VALUES (4);

  mm_testsql=# SELECT * FROM x FOR UPDATE LIMIT 1;
   id
  ----
    1
  (1 row)


Simple enough. Now, in a separate transaction, run the following

  mm_testsql=# BEGIN ;
  BEGIN 
  mm_testsql=# DELETE FROM X WHERE id = 1 ;
  DELETE 1

Select for update will now block

  mm_testsql=# SELECT * FROM x FOR UPDATE LIMIT 1;

And finally, commit in the

  mm_testsql=# COMMIT ;
  COMMIT
  mm_testsql=#

In the original transaction, the following is returned

   id
  ----
  (0 rows)