Thread: Re: [BUGS] Bug #613: Sequence values fall back to previously chec kpointed

"Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
> It seems safe to do NOT write WAL record if sequence
> LSN > system RedoRecPtr because of checkpoint started after our
> check would finish only after writing to disk sequence buffer with
> proper last_value and log_cnt (nextval keeps lock on sequence buffer).

Mmm ... maybe.  Is this safe if a checkpoint is currently in progress?
Seems like you could look at RedoRecPtr and decide you are okay, but you
really are not if checkpointer has already dumped sequence' disk
buffer and will later set RedoRecPtr to a value beyond the old LSN.
In that case you should have emitted a WAL record ... but you didn't.

Considering that we've found two separate bugs in this stuff in the past
week, I think that we ought to move in the direction of making it
simpler and more reliable, not even-more-complicated.  Is it really
worth all this trouble to avoid making a WAL record for each nextval()
call?
        regards, tom lane

I said:
> Mmm ... maybe.  Is this safe if a checkpoint is currently in progress?
> Seems like you could look at RedoRecPtr and decide you are okay, but you
> really are not if checkpointer has already dumped sequence' disk
> buffer and will later set RedoRecPtr to a value beyond the old LSN.

Oh, wait, I take that back: the checkpointer advances RedoRecPtr
*before* it starts to dump disk buffers.

I'm still worried about whether we shouldn't try to simplify, rather
than add complexity.
        regards, tom lane