Index scan vs indexonly scan method

Hi,
I was working on this simple query

select field1 from table A
where A.field2 <= some_value
order by 1 desc limit some_value

and I saw that:

1) the planner on this query uses an index only scan method:

select field1 from table A
where A.field2 <= '2014-08-13 10:20:59.99648+02'
order by 1 desc limit 100

2) the planner on this query uses a classic index scan method:

select field1 from table A
where A.field2 <= '2014-08-13 10:20:59.99648+02'
order by 1 desc limit 1

the only difference between the two queries is the limit clause,
for the first query the limit is 100 and for the second the limit is 1

it seems a little bit strange...someone can help me to understand why?

My develop PostgreSQL version is a 9.4 beta

regards,
Enrico

--
That's one small step for man; one giant leap for mankind

Enrico Pirozzi
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