Re: Fix premature xmin advancement during fast forward decoding

On Fri, Apr 25, 2025 at 4:42 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Apr 25, 2025 at 10:46 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > What I'm concerned about is the back branches. With this approach all
> > back branches will have such degradations and it doesn't make sense to
> > me to optimize SnapBuildCommitTxn() codes in back branches.
> >
>
> One possibility could be that instead of maintaining an entire
> snapshot in fast_forward mode, we can maintain snapshot's xmin in each
> ReorderBufferTXN. But then also, how would we get the minimum
> txns_by_base_snapshot_lsn as we are getting now in
> ReorderBufferGetOldestXmin? I think we need to traverse the entire
> list of txns to get it in fast_forward mode but that may not show up
> because it will not be done for each transaction. We can try such a
> thing, but it won't be clean to have fast_forward specific code and
> also it would be better to add such things only for HEAD.

Agreed.

> Can you think of any better ideas?

No idea. Hmm, there seems no reasonable way to fix this issue for back
branches. I consented to the view that these costs were something that
we should have paid from the beginning.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com