Cost of max

I build a little test table with columns id serial, a text, b text. Why the cost of max(id) is made as a seq scan, as
isfor count(id)? This is a problem with very large tables. 
IMHO, select max(id) could be done using the index prova_id_key.
Your opinion? TIA. Stefano

mydb=> explain select max(id) from prova;
NOTICE:  QUERY PLAN:

Aggregate  (cost=43.00 rows=1000 width=4)
  ->  Seq Scan on prova  (cost=43.00 rows=1000 width=4)

EXPLAIN
mydb=> explain select count(id) from prova;
NOTICE:  QUERY PLAN:

Aggregate  (cost=43.00 rows=1000 width=4)
  ->  Seq Scan on prova  (cost=43.00 rows=1000 width=4)

--
Dott. Stefano Bargioni
Biblioteca della Pontificia Universita' della Santa Croce - Roma
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