Re: Significance of backend_xmin in pg_stat_activity view

Thank you for the clarification.

On Tue, Apr 7, 2020, 11:57 AM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote:

> On Mon, 2020-04-06 at 23:11 +0530, postgann2020 s wrote:
> > Could someone please explain what is the significance of backend_xmin in
> pg_stat_activity view.
> >
> > Is xmin same as backend_xmin?
>
> It is the backend's xmin horizon.
>
> That means that it is the oldest transaction ID whose effects may not be
> visible to the transaction running in the backend.
>
> Since transaction IDs are stored in each row to determine its visibility,
> the minimum of the "backend_xmin" of all backends determines the cut-off
> point beyond which all backends will agree on the visibility of tuples.
>
> This is for example relevant for VACUUM: it cannot remove any dead tuples
> that contain a transaction ID that is not older than any backend's
> "backend_xmin" (unless the tuple is "frozen", but that leads too far).
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>
>