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Re: Visibility map, partial vacuums

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Visibility map, partial vacuums
Date: 2008-10-28 17:02:39
Message-ID: 490745AF.2090602@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs wrote:
> On Tue, 2008-10-28 at 14:57 +0200, Heikki Linnakangas wrote:
>> Simon Riggs wrote:
>>> On Mon, 2008-10-27 at 14:03 +0200, Heikki Linnakangas wrote:
>>>> One option would be to just ignore that problem for now, and not 
>>>> WAL-log.
>>> Probably worth skipping for now, since it will cause patch conflicts if
>>> you do. Are there any other interactions with Hot Standby? 
>>>
>>> But it seems like we can sneak in an extra flag on a HEAP2_CLEAN record
>>> to say "page is now all visible", without too much work.
>> Hmm. Even if a tuple is visible to everyone on the master, it's not 
>> necessarily yet visible to all the read-only transactions in the slave.
> 
> Never a problem. No query can ever see the rows removed by a cleanup
> record, enforced by the recovery system.

Yes, but there's a problem with recently inserted tuples:

1. A query begins in the slave, taking a snapshot with xmax = 100. So 
the effects of anything more recent should not be seen.
2. Transaction 100 inserts a tuple in the master, and commits
3. A vacuum comes along. There's no other transactions running in the 
master. Vacuum sees that all tuples on the page, including the one just 
inserted, are visible to everyone, and sets PD_ALL_VISIBLE flag.
4. The change is replicated to the slave.
5. The query in the slave that began at step 1 looks at the page, sees 
that the PD_ALL_VISIBLE flag is set. Therefore it skips the visibility 
checks, and erroneously returns the inserted tuple.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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