Re: Make HeapTupleSatisfiesMVCC more concurrent

On 19 August 2015 at 16:21, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I wrote:
> > Andres Freund <andres(at)anarazel(dot)de> writes:
> >> I'm not sure about it, but it might be worthwhile to add a
> >> TransactionIdIsKnownCompleted() check before the more expensive parts of
> >> XidInMVCCSnapshot(). Neither the array search nor, much more so, the
> >> subtrans lookups are free.
>
> > Hmmm... the comment for TransactionIdIsKnownCompleted says it's to
> > short-circuit calls of TransactionIdIsInProgress, which we wouldn't be
> > doing anymore. Maybe it's useful anyway but I'm not convinced.
>
> After further thought, the right way to implement the equivalent
> optimization would be to add a couple of fields to struct Snapshot that
> would cache the xid last checked against that snapshot and the outcome of
> that check; this would be independent of TransactionIdIsKnownCompleted.
> There would be no need to use that cache for xids below xmin or above
> xmax, which would improve its chance of being applicable to in-doubt xids.
>
> Not entirely sure it's worth doing, but if someone wants to do the
> legwork, this would be an independent optimization possibility.

This is what I suggested upthread. It's not a massive gain, but its cheap
and a straightforward patch.

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