explain and index scan

Hi all.

PostgreSQL 9.1.2 on i686-pc-linux-gnu, compiled by gcc-4.4.real (Debian 4.4.5-8) 4.4.5, 32-bit

id | integer | not null Vorgabewert nextval('a_id_seq'::regclass)
a | integer | not null
b | integer | not null
Indexe:
"a_pkey" PRIMARY KEY, btree (id)
"a_a_key" UNIQUE CONSTRAINT, btree (a, b)

explain select id from a where a = 1 and b = -90875 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------
Index Scan using a_a_key on a (cost=0.00..2.37 rows=1 width=4)
Index Cond: ((a = 1) AND (b = (-90875)))

explain select id from a where b = -90875 ;
QUERY PLAN
---------------------------------------------------------------------------------------------------
Index Scan using a_a_key on a (cost=0.00..961.76 rows=1 width=4)
Index Cond: (b = (-90875))

Both select where shown as 'Index Scan'. But the second select is not a real index scan,
its more a seq scan on an index, i think. I think, it would be a good idea to show this in the
explain. Now you can see this only if you look at the cost.

Best regards and thank you for your work,
Andreas