Re: Advance xmin aggressively on Read Commit isolation level

On Fri, Nov 6, 2020 at 9:48 PM Andy Fan <zhihui.fan1213@gmail.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 :-)