Wrong index used when ORDER BY LIMIT 1

Dear Gurus,

Version: 7.4.6

I use a query on a heavily indexed table which picks a wrong index
unexpectedly. Since this query is used in response to certain user
interactions thousands of times in succession (with different constants),
500ms is not affordable for us. I can easily work around this, but I'd like
to understand the root of the problem.

Basically, there are two relevant indexes:
- muvelet_vonalkod_muvelet btree (muvelet, ..., idopont)
- muvelet_vonalkod_pk3 btree (idopont, ...)

Query is:
SELECT idopont WHERE muvelet = x ORDER BY idopont LIMIT 1.

I expected the planner to choose the index on muvelet, then sort by idopont.
Instead, it took the other index. I think there is heavy correlation since
muvelet references to a sequenced pkey and idopont is a timestamp (both
increase with passing time). May that be a cause?

See full table description and explain analyze results at end of the email.

TIA,
--
G.

---- table :
Table "public.muvelet_vonalkod"
Column | Type | Modifiers
------------+--------------------------+-----------------------------------
az | integer | not null def. nextval('...')
olvaso_nev | character varying | not null
vonalkod | character varying | not null
mozgasnem | integer | not null
idopont | timestamp with time zone | not null
muvelet | integer |
minoseg | integer | not null
cikk | integer |
muszakhely | integer |
muszakkod | integer |
muszaknap | date |
repre | boolean | not null default false
hiba | integer | not null default 0
Indexes:
"muvelet_vonalkod_pkey" primary key, btree (az)
"muvelet_vonalkod_pk2" unique, btree (olvaso_nev, idopont)
"muvelet_vonalkod_muvelet" btree
(muvelet, mozgasnem, vonalkod, olvaso_nev, idopont)
"muvelet_vonalkod_pk3" btree (idopont, olvaso_nev)
"muvelet_vonalkod_vonalkod" btree
(vonalkod, mozgasnem, olvaso_nev, idopont)
Foreign-key constraints:
"$1" FOREIGN KEY (mozgasnem) REFERENCES mozgasnem(az)
"$2" FOREIGN KEY (muvelet) REFERENCES muvelet(az)
"$3" FOREIGN KEY (minoseg) REFERENCES minoseg(az)
"$4" FOREIGN KEY (cikk) REFERENCES cikk(az)
"$5" FOREIGN KEY (muszakhely) REFERENCES hely(az)
"$6" FOREIGN KEY (muszakkod) REFERENCES muszakkod(az)
"muvelet_vonalkod_muszak_fk"
FOREIGN KEY (muszakhely, muszaknap, muszakkod)
REFERENCES muszak(hely, nap, muszakkod)
Triggers:
muvelet_vonalkod_aiud AFTER INSERT OR DELETE OR UPDATE ON
muvelet_vonalkod FOR EACH ROW EXECUTE PROCEDURE muvelet_vonalkod_aiud()
muvelet_vonalkod_biu BEFORE INSERT OR UPDATE ON muvelet_vonalkod FOR
EACH ROW EXECUTE PROCEDURE muvelet_vonalkod_biu()
muvelet_vonalkod_noty AFTER INSERT OR DELETE OR UPDATE ON
muvelet_vonalkod FOR EACH ROW EXECUTE PROCEDURE muvelet_vonalkod_noty()

-- original query, limit
# explain analyze
select idopont from muvelet_vonalkod
where muvelet=6859 order by idopont
limit 1;
QUERY PLAN

----------------------------------------------------------------------------
Limit (cost=0.00..25.71 rows=1 width=8) (actual time=579.528..579.529
rows=1 loops=1)
-> Index Scan using muvelet_vonalkod_pk3 on muvelet_vonalkod
(cost=0.00..8304.42 rows=323 width=8) (actual time=579.522..579.522 rows=1
loops=1)
Filter: (muvelet = 6859)
Total runtime: 579.606 ms
(4 rows)

-- however, if I omit the limit clause:
# explain analyze
select idopont from muvelet_vonalkod
where muvelet=6859 order by idopont;
QUERY PLAN

---------------------------------------------------------------------------
Sort (cost=405.41..405.73 rows=323 width=8) (actual time=1.295..1.395
rows=360 loops=1)
Sort Key: idopont
-> Index Scan using muvelet_vonalkod_muvelet on muvelet_vonalkod
(cost=0.00..400.03 rows=323 width=8) (actual time=0.049..0.855 rows=360 loops=1)
Index Cond: (muvelet = 6859)
Total runtime: 1.566 ms
(5 rows)

-- workaround 1: the planner is hard to trick...
# explain analyze
select idopont from
(select idopont from muvelet_vonalkod
where muvelet=6859) foo
order by idopont limit 1;
QUERY PLAN

---------------------------------------------------------------------------
Limit (cost=0.00..25.71 rows=1 width=8) (actual time=584.403..584.404
rows=1 loops=1)
-> Index Scan using muvelet_vonalkod_pk3 on muvelet_vonalkod
(cost=0.00..8304.42 rows=323 width=8) (actual time=584.397..584.397 rows=1
loops=1)
Filter: (muvelet = 6859)
Total runtime: 584.482 ms
(4 rows)

-- workaround 2: quite ugly but seems to work (at least for this
-- one test case):
# explain analyze
select idopont from
(select idopont from muvelet_vonalkod
where muvelet=6859 order by idopont) foo
order by idopont limit 1;
QUERY PLAN

---------------------------------------------------------------------------
Limit (cost=405.41..405.42 rows=1 width=8) (actual time=1.754..1.755
rows=1 loops=1)
-> Subquery Scan foo (cost=405.41..407.35 rows=323 width=8) (actual
time=1.751..1.751 rows=1 loops=1)
-> Sort (cost=405.41..405.73 rows=323 width=8) (actual
time=1.746..1.746 rows=1 loops=1)
Sort Key: idopont
-> Index Scan using muvelet_vonalkod_muvelet on
muvelet_vonalkod (cost=0.00..400.03 rows=323 width=8) (actual
time=0.377..1.359 rows=360 loops=1)
Index Cond: (muvelet = 6859)
Total runtime: 1.853 ms
(7 rows)