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
Tel. +39 0861 1855771 - Mob.+39 328 4164437 - Fax +39 0861 1850310
http://www.pgtraining.com <http://www.pgtraining.com-> -
info(at)pgtraining(dot)com
www.enricopirozzi.info - info(at)enricopirozzi(dot)info
PEC: enrico(dot)pirozzi(at)pec(dot)it
Skype sscotty71 - Gtalk sscotty71(at)gmail(dot)com