Identity not disvoered by planner?

I have an hierarchical table which I join on id = parent.

I'm surprised that the planner doesn't seem to notice that parent.id =
child.parent and parent.id = 1193 implies that child.parent = 1193. As displayed
below it takes different paths when I explicitly restrict the query on both
keys or only on the parent. Furthermore, it takes the same path, but estimates
the cost differently when I restrict on both or only on the child rows.

(The query below is simplified to illustrate the plan. I need the join in the
real query.)

I'm running 7.2.1 on Red Hat 7.2.

1. Query only restricted on parent.id:

easytest=# explain select
easytest-#   parent.id,
easytest-#   count(*)
easytest-# from
easytest-#   t_object parent,
easytest-#   t_object child
easytest-# where
easytest-#   parent.id = child.parent and
easytest-#   parent.id = 1193
easytest-# group by
easytest-#   parent.id
easytest-# ;
NOTICE:  QUERY PLAN:

Aggregate  (cost=18.82..18.82 rows=1 width=8) ->  Group  (cost=18.82..18.82 rows=1 width=8)       ->  Sort
(cost=18.82..18.82rows=1 width=8)             ->  Hash Join  (cost=5.40..18.81 rows=1 width=8) ->  Seq Scan on t_object
child (cost=0.00..11.60 rows=360 width=4)                   ->  Hash  (cost=5.39..5.39 rows=1 width=4)
      ->  Index Scan using t_object_pkey on t_object parent  (cost=0.00..5.39 rows=1 width=4)
 

EXPLAIN

2. Query restricted both on parent.id and child.parent:

easytest=# explain select
easytest-#   parent.id,
easytest-#   count(*)
easytest-# from
easytest-#   t_object parent,
easytest-#   t_object child
easytest-# where
easytest-#   parent.id = child.parent and
easytest-#   parent.id = 1193 and
easytest-#   child.parent = 1193
easytest-# group by
easytest-#   parent.id
easytest-# ;
NOTICE:  QUERY PLAN:

Aggregate  (cost=0.00..10.02 rows=1 width=8) ->  Group  (cost=0.00..10.01 rows=1 width=8)       ->  Nested Loop
(cost=0.00..10.01rows=1 width=8)             ->  Index Scan using t_object_pkey on t_object parent (cost=0.00..5.39
rows=1width=4)             ->  Index Scan using xt_object_parent on t_object child (cost=0.00..4.60 rows=1 width=4)
 

EXPLAIN

3. Query restricted only on child.parent:

easytest=# explain select
easytest-#   parent.id,
easytest-#   count(*)
easytest-# from
easytest-#   t_object parent,
easytest-#   t_object child
easytest-# where
easytest-#   parent.id = child.parent and
easytest-#   child.parent = 1193
easytest-# group by
easytest-#   parent.id
easytest-# ;
NOTICE:  QUERY PLAN:

Aggregate  (cost=0.00..18.08 rows=1 width=8) ->  Group  (cost=0.00..18.08 rows=2 width=8)       ->  Nested Loop
(cost=0.00..18.07rows=2 width=8)             ->  Index Scan using xt_object_parent on t_object child (cost=0.00..6.72
rows=2width=4)             ->  Index Scan using t_object_pkey on t_object parent (cost=0.00..5.39 rows=1 width=4)
 

EXPLAIN

/Daniel