Re: No xmin in pg_database

From the version string I can suggest that it is vanilla postgres from
The PostgreSQL Global Development Group (PGDG).
But we will ask the user.

I've checked PG source code, it checks system attrs (like xmin)
existence in some cache. Maybe that is really corruption.
Does postgres itself rely on xmins of system tables?

//offtop
As for xmin usage, we have a working scheme. We fetch objects based on
dbage(xid), starting from the oldest uncommitted transaction of
previous synchronization.
Do you think it does not work?

On Thu, Sep 2, 2021 at 11:53 AM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote:
>
> On Thu, 2021-09-02 at 08:50 +0300, Alexander Kass wrote:
> > Hi! I'm Alexander from DataGrip.
> > We actively use xmin's from pg_catalog tables to incrementally
> > synchronize our database model.
> > We have a small number of users that do not have xmin in pg_database
> > (we've asked them to try `select xmin from pg_database` and got
> > `column xmin does not exist`).
> > There is an old ticket https://youtrack.jetbrains.com/issue/DBE-7588
> > (at that time there was no real need in that xmin so we just removed
> > it)
> > Now xmins are back and so are the problems.
> > The most recent report is about version `PostgreSQL 12.4 (Ubuntu
> > 12.4-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
> > 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit`
> >
> > How can that happen that there is no xmin in pg_database?
> > Is it a normal behaviour and that is configurable? Or is this a kind
> > of data corruption?
> > Can that happen to other tables?
> >
> > I haven't been able to find answers myself, so I'm asking for help :)
> > Thanks in advance
>
> All PostgreSQL tables have "xmin", and all catalog tables do as well.
>
> If you use a non-standard table access method, a table might not
> have "xmin". But that cannot apply to catalog tables.
>
> Perhaps that was not really PostgreSQL, but some fork where the
> persistence layer was modified?
>
> I am not sure if it is a good idea to rely on "xmin" at all. These
> numbers are recycled when transaction IDs wrap around, and you could
> have two entries with the same "xmin" that have a totally different
> meaning, because one of the rows is frozen and the other isn't.
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>