Message-ID: <1510740365.2424.22.camel@cybertec.at>
Subject: Re: Vacuum not removing dead tuples
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Rui DeSousa <rui.desousa@icloud.com>, pgsql-admin@postgresql.org
Date: Wed, 15 Nov 2017 11:06:05 +0100
In-Reply-To: <7D8ED7AF-E189-4FA2-BB6D-FC9E8A127E28@icloud.com>
References: <7D8ED7AF-E189-4FA2-BB6D-FC9E8A127E28@icloud.com>
Organization: Cybertec
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Rui DeSousa wrote:
> Is there a reason vacuum will not remove dead tuples other than the usual suspects?
> There are no two phase transactions and even after a restoring the database
> from binary backup with nothing connect to the restored instance;
> vacuum still does not remove the dead tuples.

There are three possibilities:

1) Active transactions:

   SELECT backend_xmin, xact_start, datname, pid, query
   FROM pg_stat_activity
   WHERE state <> 'idle'
   ORDER BY xact_start;

2) Prepared transactions:

   SELECT xid, prepared
   FROM pg_prepared_xacts;

3) Lagging replication slots:

   SELECT xmin, slot_name, active, active_pid
   FROM pg_replication_slots;

The "xid" columns will tell you which tuples VACUUM can
remove: Any tuple with "xmax" less than this are fair game.

Yours,
Laurenz Albe


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