Re: BUG #5255: COUNT(*) returns wrong result with LEFT JOIN

"Matteo Beccati" <php(at)beccati(dot)com> writes:
> With the following data set a SELECT * query returns 1 rows, while SELECT
> COUNT(*) returns 2.

Hm, looks like the join-elimination patch is firing mistakenly. It's not
so much the count(*) that does it as the lack of any select-list
references to a:

regression=# explain SELECT * FROM b LEFT JOIN a ON (b.a_id = a.id) WHERE (a.id IS NULL OR a.id > 0);
QUERY PLAN
-----------------------------------------------------------------
Hash Left Join (cost=64.00..132.85 rows=720 width=12)
Hash Cond: (b.a_id = a.id)
Filter: ((a.id IS NULL) OR (a.id > 0))
-> Seq Scan on b (cost=0.00..31.40 rows=2140 width=8)
-> Hash (cost=34.00..34.00 rows=2400 width=4)
-> Seq Scan on a (cost=0.00..34.00 rows=2400 width=4)
(6 rows)

regression=# explain SELECT b.* FROM b LEFT JOIN a ON (b.a_id = a.id) WHERE (a.id IS NULL OR a.id > 0);
QUERY PLAN
-----------------------------------------------------
Seq Scan on b (cost=0.00..31.40 rows=2140 width=8)
(1 row)

I guess we missed something about when it's safe to do this optimization.

regards, tom lane