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Re: lifetime of the old CTID

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El día Dienstag, Juli 05, 2022 a las 10:40:40 +0200, Laurenz Albe escribió:

> On Tue, 2022-07-05 at 09:51 +0200, Matthias Apitz wrote:
> > We're using the SQL function currtid2() to get the new CTID of a row
> > when this was UPDATEd.
> > 
> > Investigating cases of failing updates, it turns out that the old CTID
> > has only a limited lifetime; one can check this with SQL:
> > 
> > sisis=# select ctid, d01gsi from d01buch where d01gsi = '0240564';
> >     ctid    |           d01gsi            
> > ------------+-----------------------------
> >  (29036,11) | 0240564
> > 
> > now I update the row and afterwards pick up the new CTID based on the
> > old one (29036,11):
> > 
> > sisis=# UPDATE d01buch set d01ort='Wolfratshausen' where d01gsi='0240564';
> > UPDATE 1
> > sisis=# SELECT currtid2('d01buch'::text, '(29036,11)'::tid);
> >  currtid2  
> > -----------
> >  (29036,7)
> > 
> > Now I go and pick up a coffe in our kitchen and check again:
> > 
> > sisis=# SELECT currtid2('d01buch'::text, '(29036,11)'::tid);
> >   currtid2  
> > ------------
> >  (29036,11)
> > 
> > i.e. the function now only returns it argument. and not the new CTID
> > anymore.
> > 
> > Why is this? And what triggers exactly that the old CTID can't be used
> > anymore?
> 
> It is probably the fault of your coffee.
> 
> Another explanation could be that the HOT chain was pruned while you were away.

I've read now about HOT and understand that autovacuum will prune the
HOT chain. But also a simple SELECT seems to remove it:

sisis=# SELECT currtid2('d01buch'::text, '(29036,11)'::tid);
 currtid2  
-----------
 (29036,7)


sisis=# select ctid, d01gsi from d01buch where d01gsi = '0240564';
   ctid    |           d01gsi            
-----------+-----------------------------
 (29036,7) | 0240564                    


sisis=# SELECT currtid2('d01buch'::text, '(29036,11)'::tid);
  currtid2  
------------
 (29036,11)


-- 
Matthias Apitz, ✉ guru@xxxxxxxxxxx, http://www.unixarea.de/ +49-176-38902045
Public GnuPG key: http://www.unixarea.de/key.pub





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