Here is a weird problem, I ran into...
I have two huge (80 million rows each) tables (a and b), with id as a PK on both of them and also an FK from b referencing a.
When I try to run a query like:
select * from a, b where a.id >= 7901288 and a.id=b.id limit 1;
The query takes *forever*. If I do select * from a,b where b.id >= 7901288 and a.id=b.id limit 1;
then it returns right away.
The query plan looks identical in both cases:
Limit (cost=0.00..12.51 rows=1 width=8)
-> Nested Loop (cost=0.00..1009772807.91 rows=80740598 width=8)
-> Index Scan using b_pkey on b (cost=0.00..375410773.29 rows=80740598 width=4)
-> Index Scan using a_pkey on a (cost=0.00..6.85 rows=1 width=4)
... which makes me think that it decides to use b as the outer table for both cases (which would obviously make it suck in the first one)... :-(
This happens on 7.2.4... I have a 7.3 database with the same schema, but it is not populated with data, so I could not test it on 7.3...
I looked at the 7.3's query plans though, and they look better to me:
Limit (cost=0.00..4.97 rows=1 width=8)
-> Nested Loop (cost=0.00..1657.34 rows=333 width=8)
-> Index Scan using b_pkey on b (cost=0.00..45.50 rows=333 width=4)
Index Cond: (id >= 7901288)
-> Index Scan using a_pkey on a (cost=0.00..4.82 rows=1 width=4)
Index Cond: (a.id = "outer".id)
in the second case, and
Limit (cost=0.00..4.97 rows=1 width=8)
-> Nested Loop (cost=0.00..1657.34 rows=333 width=8)
-> Index Scan using a_pkey on a (cost=0.00..45.50 rows=333 width=4)
Index Cond: (id >= 7901288)
-> Index Scan using b_pkey on b (cost=0.00..4.82 rows=1 width=4)
Index Cond: ("outer".id = b.id)
in the first case... (looks like it does swap them around as I expected)...
Do you know of anything that got fixed between 7.2.4 and 7.3, related to this problem?
I also noticed that changing a,b to b,a in the from clause doesn't affect anything... and (what's even more weird) even using an explicit join doesn't help:
explain select a.duns from a natural join b dm where a.id >= 7901288 limit 1;
NOTICE: QUERY PLAN:
Limit (cost=0.00..12.78 rows=1 width=8)
-> Nested Loop (cost=0.00..1023061272.15 rows=80049919 width=8)
-> Index Scan using b_pkey on b (cost=0.00..380070641.01 rows=81786784 width=4)
-> Index Scan using a_pkey on a (cost=0.00..6.86 rows=1 width=4)
:-(
Any ideas?
Thanks a lot!
Dima
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