Re: Sequence's value can be rollback after a crashed recovery.

> > The reason is because we never flush the xlog for the nextval_internal
> > for the above case.  So if
> > the system crashes, there is nothing to redo from. It can be fixed
> > with the following online change
> > code.
> >
> > @@ -810,6 +810,8 @@ nextval_internal(Oid relid, bool check_permissions)
> >                 recptr = XLogInsert(RM_SEQ_ID, XLOG_SEQ_LOG);
> >
> >                 PageSetLSN(page, recptr);
> > +
> > +               XLogFlush(recptr);
> >         }
> >
> >
> > If a user uses sequence value for some external systems,  the
> > rollbacked value may surprise them.
> > [I didn't run into this issue in any real case,  I just studied xlog /
> > sequence stuff today and found this case].
>
> I think that is a bad idea.
> It will have an intolerable performance impact on OLTP queries, doubling
> the number of I/O requests for many cases.
>

The performance argument was expected before this writing. If we look at the
nextval_interval more carefully, we can find it would not flush the xlog every
time even the sequence's cachesize is 1. Currently It happens every 32 times
on the nextval_internal at the worst case.

> Perhaps it would make sense to document that you should never rely on
> sequence values from an uncommitted transaction.

I am OK with this if more people think this is the solution.


-- 
Best Regards
Andy Fan