SET enable_seqscan = off;
CREATE TABLE t (val vector(3));
INSERT INTO t (val) VALUES ('[0,0,0]'), ('[1,2,3]'), ('[1,1,1]'), (NULL);
CREATE INDEX ON t USING ivfflat (val vector_l2_ops) WITH (lists = 1);
INSERT INTO t (val) VALUES ('[1,2,4]');
SELECT * FROM t ORDER BY val <-> '[3,3,3]';
   val   
---------
 [1,2,3]
 [1,2,4]
 [1,1,1]
 [0,0,0]
(4 rows)

SELECT * FROM t ORDER BY val <-> (SELECT NULL::vector);
   val   
---------
 [0,0,0]
 [1,1,1]
 [1,2,3]
 [1,2,4]
(4 rows)

SELECT COUNT(*) FROM t;
 count 
-------
     5
(1 row)

TRUNCATE t;
NOTICE:  ivfflat index created with little data
DETAIL:  This will cause low recall.
HINT:  Drop the index until the table has more data.
SELECT * FROM t ORDER BY val <-> '[3,3,3]';
 val 
-----
(0 rows)

DROP TABLE t;