Re: Vacuum not deleting tuples when lockless

Tom & Jerry,

Thanks a lot for information!

On Monday (weekends don't have the same load patterns compared to business days) I will take a look at ` pg_prepared_xacts` that seems to expose Jerry's suggestion on xacts. Replication slots don't apply to 9.2.X from what I could investigate so I will discard that suggestion. 

Feedback setting (hot_standby_feedback) is turned off in all our replicas, this shouldn't be an issue from what I understood. 

Delay setting (vacuum_defer_cleanup_age ) in our master is configured to 0, , this shouldn't be an issue from what I understood. 

Thanks a lot!

Best,
Martín

On Fri, Sep 14th, 2018 at 11:29 PM, Jerry Sievers <gsievers19(at)comcast(dot)net> wrote:

>
>
>
> Tom Lane < tgl(at)sss(dot)pgh(dot)pa(dot)us > writes:
>
> > =?UTF-8?q?Mart=C3=ADn_Fern=C3=A1ndez?= < fmartin91(at)gmail(dot)com > writes:
> >
> >> We are experiencing some `vacuum` issues with a given table
> >> (potentially more). When a manual vacuum runs on the given table it
> >> seems that the `vacuum` process is not doing the expected cleanup.
> >
> >> DETAIL:  113257 dead row versions cannot be removed yet.
> >
> > Locks don't really have anything to do with that: what does matter is
> > how old is the oldest open transaction, because that determines the
> > "event horizon" that dead row versions have to fall below before they
> > can be removed. That oldest transaction might not be holding any locks
> > at the moment, but it doesn't matter, because in principle it could ask
> > to read this table later --- and it should see the table's contents as
> > of its snapshot.
> >
> > Serializable transactions are worse than repeatable-read transactions
> > for this purpose, because the former will keep a snapshot as of their
> > start time.
> >
> > As Jerry mentioned, replication slots can also act like open
> transactions
> > for this purpose, though I don't recall how much of that behavior is
> > present in 9.2.x.
>
> Oops, didn't notice OP was on 9.2! Presume none, since I don't think we
> got rep slots till 9.4 :-)
>
> >
> > regards, tom lane
> >
> >
>
> --
> Jerry Sievers
> Postgres DBA/Development Consulting
> e: postgres(dot)consulting(at)comcast(dot)net
> p: 312.241.7800
>
>
>
>