Why my index does not increase performance?

Hi, All!

My database has a big table (more 1 million records) that contains
a date field. This field has approximately 150 unique values (dates
of five monthes) per table. I created index as follow:

create index mytable_ind on mytable (mydate date_ops);

Next, I perform following query:

select max(mydate) from mytable;

And time length of this query is about 100 seconds! I think postgres
looks up ALL records in the table to find max value and does not use
created index. Why? Database is vacuumed, PostgreSQL 6.3.2 on Solaris
2.5.1/x86, CPU P166, RAM 64 Mb.

Yuri.