Re: tracking commit timestamps

On 01/11/14 18:44, Andres Freund wrote:
> On 2014-11-01 22:00:40 +0900, Michael Paquier wrote:
>> On Sat, Nov 1, 2014 at 1:45 PM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
>> wrote:
>>
>>> I am still planning to do more extensive tests, and study a bit more
>>> committs.c (with even more comments) as it is the core part of the feature.
>>>
>> More comments:
>> - Heikki already mentioned it, but after reading the code I see little
>> point in having the extra field implementing like that in core for many
>> reasons even if it is *just* 4 bytes:
>> 1) It is untested and actually there is no direct use for it in core.
>
> Meh. The whole feature is only there for extensions, not core.
>
>> 2) Pushing code that we know as dead is no good, that's a feature more or
>> less defined as maybe-useful-but-we-are-not-sure-yet-what-to-do-with-it.
>
> Uh. It's not more/less dead than the whole of committs.
>
>> 3) If you're going to re-use this API in BDR, which is a fork of Postgres.
>> You'd better complete this API in BDR by yourself and not bother core with
>> that.
>
> I think that's a fundamentally wrong position. The only reason BDR isn't
> purely stock postgres is that some features couldn't sanely be made work
> without patches. I *hate* the fact that we had to do so. And I really
> hope that we don't need any of the patches we have when building against
> 9.5.
>
> So, now you might argue that the additional data is useless. But I think
> that's just not thought far enough. If you think about it, in which
> scenarios do you want to map xids to the commit timestamp? Primarily
> that's going to be replication, right? One of the most obvious usecases
> is allowing to detect/analyze/resolve conflicts in a multimaster setup,
> right? To make sensible decisisons you'll often want to have more
> information about the involved transactions. Makes sense so far?
>
> Now, you might argue that could just be done with some table
> transaction_metadata(xid DEFAULT txid_current(), meta, data). But that
> has *significant* disadvantages: For one, it'll not work correctly once
> subtransactions are used. Not good. For another it has about a
> magnitude higher overhead than the committs way.
>
> And it's not like the the extra field is in any way bdr specific - even
> if you actually want to store much more information about the
> transaction than just the origin (which is what bdr does), you can use
> it to correctly solve the subtransaction problem and refer to some
> transaction metadata table.
>

Well, Michael has point that the extradata is pretty much useless
currently, perhaps it would help to add the interface to set extradata?

--
Petr Jelinek http://www.2ndQuadrant.com/
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