order by and limit with multi-column index, bug?

as a workaround for min()/max() on indexed columns forcing an index-scan
I tried "order by" with "limit 1". Works fine for the first record
but fails for the last. I don't know why.

Here's the setup:

A table "journal" containing several million records with an index

"CREATE INDEX journal_kblattsoll ON journal
(sollkontoid,waehrungid,periode,belegdatum,journalnr)"

with periode being an INT, belegdatum DATE, the rest is INT8.

As a replacement for

"select min(periode) from journal
where sollkontoid=266122::int8 and waehrungid=17::int8"

which for some reason in the design of postgres scans the whole index,
I tried:

"select sollkontoid,waehrungid,periode,belegdatum,journalnr from journal
where sollkontoid=266122::int8 and waehrungid=17::int8
order by sollkontoid,waehrungid,periode,belegdatum,journalnr asc limit 1;"

this yields:
sollkontoid | waehrungid | periode | belegdatum | journalnr
-------------+------------+---------+------------+-----------
266122 | 17 | 0 | 2002-01-01 | 411

which is correct and works in a fraction of a second as expected.

now, doing the same with "desc" instead of "asc" should return
"periode = 12" (see below) for the last record, but it doesn't!

After a fairly long time I get:

sollkontoid | waehrungid | periode | belegdatum | journalnr
-------------+------------+---------+------------+-----------
266122 | 17 | 0 | 2002-01-01 | 2783

ooops???! periode = 0???

Query plan:
Limit (cost=491999.72..491999.73 rows=1 width=32)
-> Sort (cost=491999.72..492309.30 rows=123828 width=32)
Sort Key: sollkontoid, waehrungid, periode, belegdatum, journalnr
-> Index Scan using journal_kblattsoll on journal
(cost=0.00..481525.10 rows=123828 width=32)
Index Cond: ((sollkontoid = 266122::bigint) AND (waehrungid =
17::bigint))

Surprisingly enough, reducing the fields in the order-by clause
returns the correct value for "periode":

select sollkontoid,waehrungid,periode,belegdatum,journalnr from journal
where sollkontoid=266122::int8 and waehrungid=17::int8
order by sollkontoid,waehrungid,periode desc limit 1;

sollkontoid | waehrungid | periode | belegdatum | journalnr
-------------+------------+---------+------------+-----------
266122 | 17 | 12 | 2002-12-09 | 303609

min/max-checks:

select max(periode)from journal where sollkontoid=266122::int8 and
waehrungid=17::int8;

returns 12.

select max(belegdatum) from journal where sollkontoid=266122::int8 and
waehrungid=17::int8 and periode=12;

returns "2002-12-10"

select max(journalnr) from journal where sollkontoid=266122::int8 and
waehrungid=17::int8 and periode=12 and belegdatum='2002-12-10';

returns 305098.

Consequently, the last record according to the order by clause should be:

sollkontoid | waehrungid | periode | belegdatum | journalnr
-------------+------------+---------+------------+-----------
266122 | 17 | 12 | 2002-12-10 | 305098

questions:

- what's this???
- why does it take that long?
- is "ORDER BY ... DESC" broken? (even after dropping the index I get
the same results)
- am I missing something (friday, 13th, ...) ?

thanx for any answer,
harald.

(postgres 7.3 on redhat 8.0)