Re: [COMMITTERS] pgsql: Keep track of transaction commit timestamps

On 12/29/2014 12:39 PM, Petr Jelinek wrote:
> On 29/12/14 11:16, Andres Freund wrote:
>> On 2014-12-29 12:06:07 +0200, Heikki Linnakangas wrote:
>>> To be honest, I think this patch should be reverted. Instead, we should
>>> design a system where extensions can define their own SLRUs to store
>>> additional per-transaction information. That way, each extension can have as
>>> much space per transaction as needed, and support functions that make most
>>> sense with the information. Commit timestamp tracking would be one such
>>> extension, and for this node ID stuff, you could have another one (or
>>> include it in the replication extension).
>>
>> If somebody wants that they should develop it. But given that we, based
>> on previous discussions, don't want to run user defined code in the
>> relevant phase during transaction commit *and* replay I don't think it'd
>> be all that easy to do it fast and flexible.
>
> Right, I would love to have custom SLRUs but I don't see it happening
> given those two restrictions, otherwise I would write the CommitTs patch
> that way in the first place...

Transaction commit and replay can treat the per-transaction information
as an opaque blob. It just needs to be included in the commit record,
and replay needs to write it to the SLRU. That way you don't need to run
any user-defined code in those phases.

- Heikki