> > 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