Perhaps I was too brief before, wanting to spare folks long quotes from the manual
that we all have, but here goes (quoting from
http://www.postgresql.org/users-lounge/docs/7.0/postgres/sql-createsequence.htm):
"After a sequence is created, you may use the function nextval(seqname) to get a new
number from the sequence. The function currval('seqname') may be used to determine the
number returned by the last call to nextval(seqname) for the specified sequence _in
the current session_". [emphasis added]
"Unexpected results may be obtained if a cache setting greater than one is used for a
sequence object that will be used
concurrently by multiple backends. Each backend will allocate and cache successive
sequence values during one access to
the sequence object and increase the sequence object's last_value accordingly. Then,
the next cache-1 uses of nextval within
that backend simply return the preallocated values without touching the shared
object. So, numbers allocated but not used in
the current session will be lost. __Furthermore, although multiple backends are
guaranteed to allocate distinct sequence values,
the values may be generated out of sequence when all the backends are considered.__"
[again, emphasis added].
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