Hi Alena,
Thank you for your work on subqueries with JOIN.
Have you considered the scenario where in subquery includes a qual like
(tc.aid = 1)? When I tried executing those queries I receive different
results. In my opinion, to prevent this, we should add filters for such
quals within the loop 'foreach (lc, all_clauses)'
EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF) SELECT * FROM ta
WHERE EXISTS (SELECT * FROM tb, tc WHERE ta.id = tb.id AND tc.aid = 1);
QUERY PLAN
----------------------------------------------------------------------
Hash Join (actual rows=1 loops=1)
Hash Cond: (ta.id = tb.id)
Buffers: local hit=3
-> Seq Scan on ta (actual rows=3 loops=1)
Buffers: local hit=1
-> Hash (actual rows=3 loops=1)
Buckets: 4096 Batches: 1 Memory Usage: 33kB
Buffers: local hit=2
-> HashAggregate (actual rows=3 loops=1)
Group Key: tb.id
Batches: 1 Memory Usage: 121kB
Buffers: local hit=2
-> Nested Loop (actual rows=3 loops=1)
Buffers: local hit=2
-> Seq Scan on tb (actual rows=3 loops=1)
Buffers: local hit=1
-> Materialize (actual rows=1 loops=3)
Storage: Memory Maximum Storage: 17kB
Buffers: local hit=1
-> Seq Scan on tc (actual rows=1 loops=1)
Filter: (aid = 1)
Rows Removed by Filter: 1
Buffers: local hit=1
(23 rows)
============================
EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
SELECT * FROM ta WHERE EXISTS (SELECT * FROM tb JOIN tc ON ta.id =
tb.id WHERE tc.aid = 1);
QUERY PLAN
---------------------------------------------------------------------------
Seq Scan on ta (actual rows=1 loops=1)
Filter: EXISTS(SubPlan 1)
Rows Removed by Filter: 2
Buffers: local hit=6
SubPlan 1
-> Nested Loop (actual rows=0 loops=3)
Buffers: local hit=5
-> Index Only Scan using tb_pkey on tb (actual rows=0 loops=3)
Index Cond: (id = ta.id)
Heap Fetches: 1
Buffers: local hit=4
-> Seq Scan on tc (actual rows=1 loops=1)
Filter: (aid = 1)
Buffers: local hit=1
(14 rows)
--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.
