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Re: Vacuum not deleting tuples when lockless - Mailing list pgsql-general

From Martín Fernández
Subject Re: Vacuum not deleting tuples when lockless
Date
Msg-id 5b9d792caa20125720000002@polymail.io
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In response to Re: Vacuum not deleting tuples when lockless  (Jerry Sievers <gsievers19@comcast.net>)
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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@comcast.net> wrote:

Tom Lane <tgl@sss.pgh.pa.us> writes:

> Martín Fernández <fmartin91@gmail.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.consulting@comcast.net
p: 312.241.7800


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