What are multixactids?

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

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Andreas Karlsson

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On Mon, Dec 09, 2013 at 07:59:10PM +0100, Andreas Karlsson wrote:

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

Thanks. Read that. Still, it would be good to have some information in
normal docs, but I guess this has to do for now.

Best regards,

depesz

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On 12/09/2013 08:05 PM, hubert depesz lubaczewski wrote:

Thanks. Read that. Still, it would be good to have some information in
normal docs, but I guess this has to do for now.

It is mentioned several times in the documentation but I do not think it
is explained anywhere.

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Andreas Karlsson

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On 12/9/13 1:05 PM, hubert depesz lubaczewski wrote:

On Mon, Dec 09, 2013 at 07:59:10PM +0100, Andreas Karlsson wrote:

I recommend you read the section in README.tuplock.
1. https://github.com/postgres/postgres/blob/d9250da032e723d80bb0150b9276cc544df6a087/src/backend/access/heap/README.tuplock#L68

+1, even if it's just a link to the README. I also started wondering what these were while investigating the vacuum bugs.

I'd submit a patch, but I don't have doc builds working :(
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Jim C. Nasby, Data Architect jim@nasby.net
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