Re: logical decoding - GetOldestXmin

On 2012-12-13 17:29:06 -0500, Robert Haas wrote:
> On Thu, Dec 13, 2012 at 3:03 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> > It moves a computation of the sort of:
> >
> > result -= vacuum_defer_cleanup_age;
> > if (!TransactionIdIsNormal(result))
> >    result = FirstNormalTransactionId;
> >
> > inside ProcArrayLock. But I can't really imagine that to be relevant...
>
> I can.  Go look at some of the 9.2 optimizations around
> GetSnapshotData().  Those made a BIG difference under heavy
> concurrency and they were definitely micro-optimization.  For example,
> the introduction of NormalTransactionIdPrecedes() was shockingly
> effective.

But GetOldestXmin() should be called less frequently than
GetSnapshotData() by several orders of magnitudes. I don't really see
it being used in any really hot code paths?

Greetings,

Andres Freund

--Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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