On 19 August 2015 at 16:21, Tom Lane <t...@sss.pgh.pa.us> wrote: > I wrote: > > Andres Freund <and...@anarazel.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. -- Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
