Updates on one row causing ExclusiveLock on PostgreSQL 8.3.5

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My best regards for all...

Please.  I need for an advice.
I'm having a trouble, that puting others queries in wait state, becouse of ExclusiveLock granted by an Update that only update one row at each time. This update occurs into a function and this function are executed several times and concurrently.
Below, query plan (explain):

Nested Loop  (cost=16.91..36.32 rows=1 width=75)
  ->  HashAggregate  (cost=16.91..16.92 rows=1 width=4)
-> Index Scan using unq_customer_idx_msisdn on customer (cost=0.00..16.90 rows=1 width=4)
              Index Cond: ((msisdn)::text = '558796013980'::text)
-> Index Scan using pk_customer_rel_channel on customer_rel_channel (cost=0.00..19.39 rows=1 width=75) Index Cond: ((customer_rel_channel.id_customer = customer.id_customer) AND (customer_rel_channel.id_channel = 282))

But, the pg_locs shows:

PID Relation User Transaction Access Mode Granted Query Start Query 22569 customer_rel_channel postgres ExclusiveLock False 2013-01-10 15:54:09.308056-02 UPDATE news.customer_rel_channel SET status = $1, source = $2
            WHERE news.customer_rel_channel.id_channel = $3 AND
            news.customer_rel_channel.id_customer IN
(SELECT id_customer FROM public.customer WHERE public.customer.msisdn = $4)

I can't understand what happens here... This query can't be lock granted becouse another instance of this query already granted it. I can't understand why an update that modify one row only need an ExclusiveLock.

Thanks a lot!!



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