max(field) vs select field .. order by desc limit 1

Hi,

Which would be better to do, max() or select .. order by limit 1? Is 7.1
much better than 7.0.3 in doing the "limit 1"? I see that their EXPLAINs
are different - 7.1 has a limit cost.

(pid is a serial in the ep_posts table)

*** For Version 7.0.3
lylyeoh=# explain select max(pid) from ep_posts;
NOTICE: QUERY PLAN:

Aggregate (cost=8.50..8.50 rows=1 width=4)
-> Seq Scan on ep_posts (cost=0.00..8.00 rows=200 width=4)

EXPLAIN
lylyeoh=# explain select pid from ep_posts order by pid desc limit 1;
NOTICE: QUERY PLAN:

Index Scan Backward using ep_posts_pid_key on ep_posts (cost=0.00..26.20
rows=200 width=4)

EXPLAIN

*** For version 7.1beta4

explain select max(pid) from ep_posts;
NOTICE: QUERY PLAN:

Aggregate (cost=8.50..8.50 rows=1 width=4)
-> Seq Scan on ep_posts (cost=0.00..8.00 rows=200 width=4)

EXPLAIN
lylyeoh=> explain select pid from ep_posts order by pid desc limit 1;
NOTICE: QUERY PLAN:

Limit (cost=0.00..0.13 rows=1 width=4)
-> Index Scan Backward using ep_posts_pid_key on ep_posts
(cost=0.00..26.20 rows=200 width=4)

EXPLAIN