Re: logical decoding - GetOldestXmin

On 2012-12-14 14:01:30 -0500, Robert Haas wrote:
> On Fri, Dec 14, 2012 at 6:46 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > Just moving that tidbit inside the lock seems to be the pragmatic
> > choice. GetOldestXmin is called
> >
> > * once per checkpoint
> > * one per index build
> > * once in analyze
> > * twice per vacuum
> > * once for HS feedback messages
> >
> > Nothing of that occurs frequently enough that 5 instructions will make a
> > difference. I would be happy to go an alternative path, but right now I
> > don't see any nice one. A "already_locked" parameter to GetOldestXmin
> > seems to be a cure worse than the disease.
>
> I'm not sure that would be so bad, but I guess I question the need to
> do it this way at all. Most of the time, if you need to advertise
> your global xmin, you use GetSnapshotData(), not GetOldestXmin(), and
> I guess I'm not seeing why that wouldn't also work here. Am I dumb?

I wondered upthread whether that would be better:

On 2012-12-13 21:03:44 +0100, Andres Freund wrote:
> Another alternative to this would be to get a snapshot with
> GetSnapshotData(), copy the xmin to the logical slot, then call
> ProcArrayEndTransaction(). But that doesn't really seem to be nicer to
> me.

Not sure why I considered it ugly anymore, but it actually has a
noticeable disadvantage. GetOldestXmin is nicer is than GetSnapshotData
as the latter set a fairly new xid as xmin whereas GetOldestXmin returns
the actual current xmin horizon. Thats preferrable because it allows us
to start up more quickly. snapbuild.c can only start building a snapshot
once it has seen a xl_running_xact with oldestRunningXid >=
own_xmin. Otherwise we cannot be sure that no relevant catalog tuples
have been removed.

This also made me notice that my changes to GetSnapshotData were quite
pessimal... I do set the xmin of the new snapshot to the "logical xmin"
instead of doing it only to globalxmin/RecentGlobalXmin.

Andres

--
Andres Freund http://www.2ndQuadrant.com/
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