Re: join removal

Hello

is any reason why join removal doesn't remove useless relation b?

postgres=# \d a
Table "public.a"
Column | Type | Modifiers
--------+---------+-----------
a | integer |
Indexes:
"a_a_idx" UNIQUE, btree (a)

postgres=# \d b
Table "public.b"
Column | Type | Modifiers
--------+---------+-----------
b | integer |
Indexes:
"b_b_idx" UNIQUE, btree (b)

postgres=# explain select a from a left join b on true;
QUERY PLAN
-------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..72074.00 rows=5760000 width=4)
-> Seq Scan on a (cost=0.00..34.00 rows=2400 width=4)
-> Materialize (cost=0.00..46.00 rows=2400 width=0)
-> Seq Scan on b (cost=0.00..34.00 rows=2400 width=0)
(4 rows)

postgres=# explain select distinct a from a left join b on true;
QUERY PLAN
---------------------------------------------------------------------------------
Unique (cost=0.00..86520.25 rows=2400 width=4)
-> Nested Loop Left Join (cost=0.00..72120.25 rows=5760000 width=4)
-> Index Scan using a_a_idx on a (cost=0.00..80.25 rows=2400 width=4)
-> Materialize (cost=0.00..46.00 rows=2400 width=0)
-> Seq Scan on b (cost=0.00..34.00 rows=2400 width=0)
(5 rows)

Regards
Pavel Stehule