Yet Another newbie not understanding why an index isn't used

I have a simple table of data from temperature probes, with two indices:

hlt=> \d hltdata
Table = hltdata
+------------------+------------------+-------+
| Field | Type | Length|
+------------------+------------------+-------+
| id | int2 | 2 |
| stmp | datetime | 8 |
| raw | int2 | 2 |
| cooked | int2 | 2 |
+------------------+------------------+-------+

hlt=> \d hltdata_all
+------------------+------------------+-------+
| id | int2 | 2 |
| stmp | datetime | 8 |
+------------------+------------------+-------+
(hltdata_all is a UNIQUE index to prevent dups.)

hlt=> \d hltdata_stmp
+------------------+------------------+-------+
| stmp | datetime | 8 |
+------------------+------------------+-------+

There are 1.5 million rows. It has been VACUUMed since the indices
were initially created in the COPY, and no rows have been added
since. It would seem like a perfect opportunity for the following
SELECT to use the indices; just grab the first 10 rows. Yet it grinds
for 93 seconds on a PII 450 w/ 256M of RAM. It's only 77 seconds if I
order by stmp alone.

hlt=> EXPLAIN SELECT * FROM hltdata ORDER BY stmp, id LIMIT 10;
NOTICE: QUERY PLAN:

Sort (cost=62394.19 rows=1584824 width=14)
-> Seq Scan on hltdata (cost=62394.19 rows=1584824 width=14)

The only entry in the FAQ prompted me to do the VACUUM, but I didn't
see anything else related to this. I reckond it must be pretty basic,
though...

--
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Felix Finch: scarecrow repairman & rocket surgeon / felix(at)crowfix(dot)com
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I've found a solution to Fermat's Last Theorem but I see I've run out of room o