Re: Conflict Detection and Resolution

On Mon, Jun 17, 2024 at 8:51 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Mon, Jun 17, 2024 at 1:42 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > The difference w.r.t the existing mechanisms for holding deleted data
> > is that we don't know whether we need to hold off the vacuum from
> > cleaning up the rows because we can't say with any certainty whether
> > other nodes will perform any conflicting operations in the future.
> > Using the example we discussed,
> > Node A:
> > T1: INSERT INTO t (id, value) VALUES (1,1);
> > T2: DELETE FROM t WHERE id = 1;
> >
> > Node B:
> > T3: UPDATE t SET value = 2 WHERE id = 1;
> >
> > Say the order of receiving the commands is T1-T2-T3. We can't predict
> > whether we will ever get T-3, so on what basis shall we try to prevent
> > vacuum from removing the deleted row?
>
> The problem arises because T2 and T3 might be applied out of order on
> some nodes. Once either one of them has been applied on every node, no
> further conflicts are possible.

If we decide to skip the update whether the row is missing or deleted,
we indeed reach the same end result regardless of the order of T2, T3,
and Vacuum. Here's how it looks in each case:

Case 1: T1, T2, Vacuum, T3 -> Skip the update for a non-existing row
-> end result we do not have a row.
Case 2: T1, T2, T3 -> Skip the update for a deleted row -> end result
we do not have a row.
Case 3: T1, T3, T2 -> deleted the row -> end result we do not have a row.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com