Re: Make HeapTupleSatisfiesMVCC more concurrent

On 2015-08-18 20:36:13 -0400, Tom Lane wrote:
> I wrote:
> > Just thinking about this ... I wonder why we need to call
> > TransactionIdIsInProgress() at all rather than believing the answer from
> > the snapshot?  Under what circumstances could TransactionIdIsInProgress()
> > return true where XidInMVCCSnapshot() had not?
>
> I experimented with the attached patch, which replaces
> HeapTupleSatisfiesMVCC's calls of TransactionIdIsInProgress with
> XidInMVCCSnapshot, and then as a cross-check has all the "return false"
> exits from XidInMVCCSnapshot assert !TransactionIdIsInProgress().

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.