Re: tracking commit timestamps

On Tue, Oct 28, 2014 at 9:25 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> On 13 October 2014 10:05, Petr Jelinek <petr(at)2ndquadrant(dot)com> wrote:
>
> >> I worked bit on this patch to make it closer to committable state.
>
> > Here is updated version that works with current HEAD for the October
> > committfest.
>
> I've reviewed this and it looks good to me. Clean, follows existing
> code neatly, documented and tested.
>
> Please could you document a few things
>
> * ExtendCommitTS() works only because commit_ts_enabled can only be
> set at server start.
> We need that documented so somebody doesn't make it more easily
> enabled and break something.
> (Could we make it enabled at next checkpoint or similar?)
>
> * The SLRU tracks timestamps of both xids and subxids. We need to
> document that it does this because Subtrans SLRU is not persistent. If
> we made Subtrans persistent we might need to store only the top level
> xid's commitTS, but that's very useful for typical use cases and
> wouldn't save much time at commit.
>

Hm. What is the performance impact of this feature using the latest version
of this patch? I imagine that the penalty of the additional operations this
feature introduces is not zero, particularly for loads with lots of short
transactions.
--
Michael