The planner chooses seqscan+sort when there is an index on the sort column

Hi all,

I wonder why this happens:

- postgres: 8.1.3
- the table has ~200 million rows;
- there is a primary key on (col_1, col_2);
- the table was ANALYZEd;
- the planner chooses seqscan+sort for the following query even with
enable_seqscan=off:

select * from table order by col_1;

Isn't it supposed to choose the index scan at least when
enable_seqscan=off ? Even if it is indeed not faster to do the index
scan than seqscan+sort.

The actual plan looks like (names changed):

db=# set enable_seqscan = off;
SET
db=# explain select * from table order by col_1;
QUERY PLAN
-----------------------------------------------------------------------------------------
Sort (cost=165585198.70..166102386.06 rows=206874944 width=60)
Sort Key: col_1
-> Seq Scan on table (cost=100000000.00..104552091.44
rows=206874944 width=60)
(3 rows)

db=# \d table
Table "public.table"
Column | Type |
Modifiers
-----------------+-----------------------------+----------------------------------------------------
col_1 | bigint | not null
col_2 | bigint | not null
...
Indexes:
"pk_table" PRIMARY KEY, btree (col_1, col_2)
...

Cheers,
Csaba.