Seq scan on 10million record table.. why?

Hi all

I have a problem with a data import procedure that involve the following query:

select a,b,c,d
from big_table b
join data_sequences_table ds
on b.key1 = ds.key1 and b.key2 = ds.key2
where ds.import_id=xxxxxxxxxx

The "big table" has something like 10.000.000 records ore more
(depending on the table, there are more than one of them).
The data are uploaded in 20k record blocks, and the keys are written
on "data_sequences_table". The keys are composite (key1,key2), and
every 5-10 sequences (depending on the size of the upload) the
data_sequences_table records are deleted.
I have indexes on both the key on the big table and the import_id on
the sequence table.

the query plan evualuate like this:

Merge Join (cost=2604203.98..2774528.51 rows=129904 width=20)
Merge Cond: (((( big_table.key1)::numeric) =
data_sequences_table.key1) AND ((( big_table.key2)::numeric) =
data_sequences_table.key2))
-> Sort (cost=2602495.47..2635975.81 rows=13392135 width=20)
Sort Key: ((big_table.key1)::numeric), ((big_table.key2)::numeric)
-> Seq Scan on big_table (cost=0.00..467919.35 rows=13392135 width=20)
-> Sort (cost=1708.51..1709.48 rows=388 width=32)
Sort Key: data_sequences_table.key1, data_sequences_table.key2
-> Seq Scan on data_sequences_table (cost=0.00..1691.83
rows=388 width=32)
Filter: (import_id = 1351592072::numeric)

It executes in something like 80 seconds. The import procedure has
more than 500 occurrences of this situation. :(
Why is the big table evaluated with a seq scan? The result is 0 to
20.000 records (the query returns the records that already exists and
should be updated, not inserted).. Can I do something to speed this
up?

--
Vincenzo.
Imola Informatica

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