Re: Advance xmin aggressively on Read Commit isolation level

On Fri, Nov 6, 2020 at 4:54 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:

> On Fri, Nov 6, 2020 at 9:48 PM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
> > I have 2 ideas about this. One is in the Read Committed level, we can
> advance xmin
> > aggressively. suppose it started at t1, and complete a query at t2. the
> xmin should
> > be t1 currently. Can we advance the xmin to t2 since it is read
> committed level,
> > The data older than t2 will never be used? Another one is can we force
> to clean
> > up the old tuples which are older than xxx? If users want to access
> that,
> > we can just raise errors. Oracle uses this strategy and the error code
> is
> > ORA-01555.
>
> Hi Andy,
>
> For the second idea, we have old_snapshot_threshold which does exactly
> that since 9.6. There have been some questions about whether it works
> correctly, though: see https://commitfest.postgresql.org/30/2682/ if
> you would like to help look into that :-)
>

Hi Tomas:
This is exactly what I want and I have big interest with that. Thanks for
the information!

--
Best Regards
Andy Fan