Strange query plan

Tom,

I have a problem with slow query execution (postgresql 7.1.2):

There are 2 tables - idx, msg_prt:
bug=# \dt
List of relations
Name | Type | Owner
---------+-------+--------
idx | table | megera
msg_prt | table | megera
(2 rows)

bug=# \d idx
Table "idx"
Attribute | Type | Modifier
-----------+---------+----------
tid | integer |
lid | integer |
did | integer |
Index: idxidx

bug=# \d msg_prt
Table "msg_prt"
Attribute | Type | Modifier
-----------+---------+----------
tid | integer |
Index: mprt_tid

Also there are 2 indexes - idxidx, mprt_tid

bug=# \d idxidx
Index "idxidx"
Attribute | Type
-----------+---------
lid | integer
did | integer
tid | integer
unique btree

bug=# \d mprt_tid
Index "mprt_tid"
Attribute | Type
-----------+---------
tid | integer
unique btree

Query is:
select msg_prt.tid as mid from msg_prt
where exists
(select idx.tid from idx where msg_prt.tid=idx.tid
and idx.did=1 and idx.lid in (1207,59587) )

Plan for this query looks very ineffective and query is very slow:

select msg_prt.tid as mid from msg_prt
where exists (select idx.tid from idx where msg_prt.tid=idx.tid
and idx.did=1 and idx.lid in (1207,59587) )
NOTICE: QUERY PLAN:

Seq Scan on msg_prt (cost=0.00..119090807.13 rows=69505 width=4)
SubPlan
-> Index Scan using idxidx, idxidx on idx (cost=0.00..1713.40 rows=1 width=4)

total: 6.80 sec; number: 1; for one: 6.796 sec;

Statistics on tables:
idx - 103651 rows
msg_prt - 69505 rows

There are only 16 rows in 'idx' table satisfied subselect condition.
I did vacuum analyze.

Adding another index 'create index tididx on idx (tid);' helps:
select msg_prt.tid as mid from msg_prt
where exists (select idx.tid from idx where msg_prt.tid=idx.tid
and idx.did=1 and idx.lid in (1207,59587) )
NOTICE: QUERY PLAN:

Seq Scan on msg_prt (cost=0.00..1134474.94 rows=69505 width=4)
SubPlan
-> Index Scan using tididx on idx (cost=0.00..16.31 rows=1 width=4)

total: 1.71 sec; number: 1; for one: 1.711 sec;

but still plan looks ineffective.

The best plan I've got eliminating IN predicate:
select msg_prt.tid as mid from msg_prt
where exists (select idx.tid from idx where msg_prt.tid=idx.tid
and idx.did=1 and idx.lid = 1207 and idx.lid=59587 )
NOTICE: QUERY PLAN:

Seq Scan on msg_prt (cost=0.00..167368.47 rows=69505 width=4)
SubPlan
-> Index Scan using idxidx on idx (cost=0.00..2.39 rows=1 width=4)

total: 0.54 sec; number: 1; for one: 0.541 sec;

Unfortunately I can't use this way in general case.

Does it's a known problem ?

data+schema is available from
http://www.sai.msu.su/~megera/postgres/data/bug.dump.gz
It's about 500Kb !

Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83