Date: Fri, 27 Nov 2015 17:47:57 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Kouber Saparev <kouber@gmail.com>, pgsql-admin@postgresql.org
Subject: Re: VACUUM unable to accomplish because of a non-existent MultiXactId
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Tom Lane wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > Tom Lane wrote:
> >> Couldn't creation and deletion of a toasted object within the same
> >> transaction do it?
> 
> > I tried and couldn't find a way to cause one to appear.  When a row is
> > created, it has xmax=0 so when it's deleted the xmax can just be set to
> > the deleting xact.  And we don't ever update pg_toast rows, AFAIK.
> 
> Ah, nevermind, brain fade ... I was momentarily confusing this with
> combo CIDs.  Right, there are no updates or row locks taken on toast
> table rows (unless someone were to do one manually?)

Both UPDATE and SELECT FOR UPDATE fail right away:

alvherre=# update pg_toast.pg_toast_16398 set chunk_id = 7 where chunk_id = 5;
ERROR:  cannot change TOAST relation "pg_toast_16398"

alvherre=# select * from pg_toast.pg_toast_16398 for update;
ERROR:  cannot lock rows in TOAST relation "pg_toast_16398"

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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