On 12/09/2013 06:04 PM, hubert depesz lubaczewski wrote:
Hi,
when working on fixing the bug related to vacuum freeze, I found out
that there is something called "MultiXactId".
Searching docs showed that it is mentioned only once, in release notes
to 9.3.2:
http://www.postgresql.org/search/?u=%2Fdocs%2F9.3%2F&q=multixactid
What's more - I found that Peter Eisentraut already once asked about
them, and lack of documentation:
http://postgresql.1045698.n5.nabble.com/MultiXactId-concept-underdocumented-td5766754.html
So, my question is - what are multixactids, what are they used for,
where can I find any documentation/explanation/whatever?
It seems to be related in some way to the relfrozenxid/vacuum bug, but
I can't comprehend the relation without knowing what multixactid
actually is.
As you can see from Peter's message it is explained in
README.tuplock[1]. Basically it is used whenever more than one lock is
acquired on the same tuples as a reference to where the locks are
stored. It can store updated/deleted Xid for the tuple so it needs to be
persisted.
I recommend you read the section in README.tuplock.
1.
https://github.com/postgres/postgres/blob/d9250da032e723d80bb0150b9276cc544df6a087/src/backend/access/heap/README.tuplock#L68
--
Andreas Karlsson
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
