Select from sequence in slow query log

[Chris Rimmer <chrisr@xxxxxxx>]

While investigating some performance issues I have been looking at slow queries logged to the postgresql.log file.  A strange thing that I have seen is a series of apparently very slow queries that just select from a sequence. It is as if access to a sequence is blocked for many sessions and then released as I get log entries like this appearing:

LOG:  duration: 23702.553 ms  execute <unnamed>: /* dynamic native SQL query */ select nextval ('my_sequence') as nextval

LOG:  duration: 23673.068 ms  execute <unnamed>: /* dynamic native SQL query */ select nextval ('my_sequence') as nextval

LOG:  duration: 23632.729 ms  execute <unnamed>: /* dynamic native SQL query */ select nextval ('my_sequence') as nextval

....(Many similar lines)....

LOG:  duration: 3055.057 ms  execute <unnamed>: /* dynamic native SQL query */ select nextval ('my_sequence') as nextval

LOG:  duration: 2377.621 ms  execute <unnamed>: /* dynamic native SQL query */ select nextval ('my_sequence') as nextval

LOG:  duration: 743.732 ms  execute <unnamed>: /* dynamic native SQL query */ select nextval ('my_sequence') as nextval

The code is being executed via Hibernate, but using Session.createSQLQuery(), so the SQL above appears in the source as above (minus the comment) and not as part of any ORM magic. We are using Postgresql 9.0.

This seems very strange to me. What could cause a sequence to be locked for such a long time?

The sequence in question has cache set at 1. Would setting this higher make any difference? 

Thanks

Chris