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)
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