Re: Sequences in transaction context

"Erik Pearson" <erik(at)cariboulake(dot)com> writes:
> Thanks for the reply! I'm afraid I didn't test this thoroughly. I noticed
> that nextval changed the sequence for all transactions, and assumed that
> currval simply translated to the (nextval() - 1), which it does not, as I
> now understand it.

No. Each backend process caches the last nextval() result it got for
every sequence object it's ever nextval'd, and this data is what
currval() consults. That's the only way to ensure cross-process
consistency. What you see in currval is the last value you obtained,
regardless of what anyone else is doing.

> One last followup question -- what's MVCC?

Multi-version concurrency control. See
http://www.ca.postgresql.org/users-lounge/docs/7.1/postgres/mvcc.html
or my slides on Postgres' transaction model at
http://www.ca.postgresql.org/osdn/index.html

regards, tom lane