Blocking excessively in FOR UPDATE - Mailing list pgsql-performance

Hi list,

I've been experiencing a weird performance issue lately.

I have a very simple (and usually very fast) query:

SELECT track_logs.id
FROM track_logs
WHERE track_logs.track_id = <some id> AND track_logs.track_status_id =
1 AND track_logs.date >= now() - interval '1 hours'
FOR UPDATE

Whose plan is:

"LockRows  (cost=0.00..26.73 rows=1 width=14)"
"  ->  Index Scan using idx_track_logs_track_id on track_logs
(cost=0.00..26.72 rows=1 width=14)"
"        Index Cond: (track_id = <some id>)"
"        Filter: ((track_status_id = 1) AND (date >= (now() -
'01:00:00'::interval)))"

The same query, without FOR UPDATE, takes just 68 milliseconds.

With the FOR UPDATE, it takes like half a minute or more to finish.

Now, I understand the for update part may be blocking on some other
transaction, and it's probably the case.
But I cannot figure out which transaction it would be. There *are*, in
fact, connections in <idle in transaction> state, which makes me think
those would be the culprit. But for the life of me, I cannot make
sense of the pg_locks view, which shows all locks as granted:


PID           Relation    XID            TX            Mode            Granted     Start
14751    5551986                154/4038460    AccessShareLock    Yes        2011-11-03 12:45:03.551516-05
14751    5526310                154/4038460    RowShareLock        Yes        2011-11-03 12:45:03.551516-05
14751    5552008                154/4038460    RowExclusiveLock    Yes        2011-11-03 12:45:03.551516-05
14751    5552020                154/4038460    RowExclusiveLock    Yes        2011-11-03 12:45:03.551516-05
14751    5552008                154/4038460    AccessShareLock    Yes        2011-11-03 12:45:03.551516-05
14751    5525296                154/4038460    RowShareLock        Yes        2011-11-03 12:45:03.551516-05
14751    5525292                154/4038460    RowShareLock        Yes        2011-11-03 12:45:03.551516-05
14751    5552019                154/4038460    AccessShareLock    Yes        2011-11-03 12:45:03.551516-05
14751    5552019                154/4038460    RowExclusiveLock    Yes        2011-11-03 12:45:03.551516-05
14751    5552020                154/4038460    AccessShareLock    Yes        2011-11-03 12:45:03.551516-05
14751    5525292                154/4038460    RowExclusiveLock    Yes        2011-11-03 12:45:03.551516-05
14751            154/4038460    154/4038460    ExclusiveLock        Yes        2011-11-03
12:45:03.551516-05
14751                        154/4038460    ExclusiveLock        Yes        2011-11-03 12:45:03.551516-05
14751    5526308                154/4038460    AccessShareLock    Yes        2011-11-03 12:45:03.551516-05

Where should I look?
What other information should I provide?