On Mon, 2021-11-22 at 15:43 +0800, Andy Fan wrote: > > > 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.
Right, I didn't think of that. Still, I'm -1 on this performance regression. Yours, Laurenz Albe