Re: [PATCH 10/16] Introduce the concept that wal has a 'origin' node

On 20.06.2012 17:35, Simon Riggs wrote:
> On 20 June 2012 16:23, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>> On 20.06.2012 11:17, Simon Riggs wrote:
>>>
>>> On 20 June 2012 15:45, Heikki Linnakangas
>>> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>>>
>>>> So, if the origin id is not sufficient for some conflict resolution
>>>> mechanisms, what extra information do you need for those, and where do
>>>> you put it?
>>>
>>> As explained elsewhere, wal_level = logical (or similar) would be used
>>> to provide any additional logical information required.
>>>
>>> Update and Delete WAL records already need to be different in that
>>> mode, so additional info would be placed there, if there were any.
>>>
>>> In the case of reflexive updates you raised, a typical response in
>>> other DBMS would be to represent the query
>>> UPDATE SET counter = counter + 1
>>> by sending just the "+1" part, not the current value of counter, as
>>> would be the case with the non-reflexive update
>>> UPDATE SET counter = 1
>>>
>>> Handling such things in Postgres would require some subtlety, which
>>> would not be resolved in first release but is pretty certain not to
>>> require any changes to the WAL record header as a way of resolving it.
>>> Having already thought about it, I'd estimate that is a very long
>>> discussion and not relevant to the OT, but if you wish to have it
>>> here, I won't stop you.
>>
>>
>> Yeah, I'd like to hear briefly how you would handle that without any further
>> changes to the WAL record header.
>
> I already did:
>
>>> Update and Delete WAL records already need to be different in that
>>> mode, so additional info would be placed there, if there were any.
>
> The case you mentioned relates to UPDATEs only, so I would suggest
> that we add that information to a new form of update record only.
>
> That has nothing to do with the WAL record header.

Hmm, so you need the origin id in the WAL record header to do filtering.
Except when that's not enough, you add some more fields to heap update
and delete records.

Don't you think it would be simpler to only add the extra fields to heap
insert, update and delete records, and leave the WAL record header
alone? Do you ever need extra information on other record types?

The other question is, *what* information do you need to put there? We
don't necessarily need to have a detailed design of that right now, but
it seems quite clear to me that we need more flexibility there than this
patch provides, to support more complicated conflict resolution.

I'm not saying that we need to implement all possible conflict
resolution algorithms right now - on the contrary I think conflict
resolution belongs outside core - but if we're going to change the WAL
record format to support such conflict resolution, we better make sure
the foundation we provide for it is solid.

BTW, one way to work around the lack of origin id in the WAL record
header is to just add an origin-id column to the table, indicating the
last node that updated the row. That would be a kludge, but I thought
I'd mention it..

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