Optimize TransactionIdIsCurrentTransactionId().

If the passed in xid is the current top transaction, we can do a fast
check and exit early.  This should work well for the current heap but
also works very well for proposed AMs that don't use a separate xid
for subtransactions.

Author: Ashwin Agrawal, based on a suggestion from Andres Freund
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CALfoeiv0k3hkEb3Oqk%3DziWqtyk2Jys1UOK5hwRBNeANT_yX%2Bng%40mail.gmail.com

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index fc55fa6d53c623cf8edddf1dce971db08c005b7b..e5e2902465aca295f631ff44d9a3796c9fcd8787 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -871,6 +871,9 @@ TransactionIdIsCurrentTransactionId(TransactionId xid)
        if (!TransactionIdIsNormal(xid))
                return false;
 
+       if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
+               return true;
+
        /*
         * In parallel workers, the XIDs we must consider as current are stored in
         * ParallelCurrentXids rather than the transaction-state stack.  Note that