Hi,
I am seeing different results with two queries which AFAIU have same semantics and hence are expected to give same results.
postgres=# \d t1
Table "public.t1"
Column | Type | Modifiers
--------+---------+-----------
val | integer |
val2 | integer |
postgres=# \d t2
Table "public.t2"
Column | Type | Modifiers
--------+---------+-----------
val | integer |
val2 | integer |
There's no data in the table to start with.
postgres=# insert into t1 values (1, 1);
postgres=# insert into t2 values (1, 1), (2, 2);
Session 1
postgres=# begin;
BEGIN
postgres=# update t1 set val = 2 where val2 = 1;
UPDATE 1
Session 2
postgres=# select * from t1, (select distinct val, val2 from t2) t2 where t1.val = t2.val for update of t1;
query waits here because of FOR UPDATE clause
Session 1
postgres=# commit;
COMMIT
Session 2 gives no rows
postgres=# select * from t1, (select distinct val, val2 from t2) t2 where t1.val = t2.val for update of t1;
val | val2 | val | val2
-----+------+-----+------
(0 rows)
Reset values of t1
postgres=# update t1 set val = 1 where val2 = 1;
UPDATE 1
Session 1
postgres=# begin;
BEGIN
postgres=# update t1 set val = 2 where val2 = 1;
UPDATE 1
Session 2
postgres=# select * from t1, lateral (select distinct val, val2 from t2 where t2.val = t1.val) t2 for update of t1;
query waits here
Session 1
postgres=# commit;
COMMIT
Session 2 gives results of the query
postgres=# select * from t1, lateral (select distinct val, val2 from t2 where t2.val = t1.val) t2 for update of t1;
val | val2 | val | val2
-----+------+-----+------
2 | 1 | 1 | 1
(1 row)
AFAIU, both the queries
select * from t1, (select distinct val, val2 from t2) t2 where t1.val = t2.val for update of t1;
AND
select * from t1, lateral (select distinct val, val2 from t2 where t2.val = t1.val) t2 for update of t1;
have same semantic and should give same results.
Is seeing different results expected behaviour?
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company