Optimize PredicateLockTuple().

PredicateLockTuple() has a fast exit if tuple was written by the current
transaction, as in that case it already has a lock.  This check can be
performed using TransactionIdIsCurrentTransactionId() instead of
SubTransGetTopmostTransaction(), to avoid any chance of having to hit the
disk.

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/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 78fb90fb1bd6e655fc8b701ca3ed1cde60688c3a..86b46d68b9933cb373e3b61af4053b9ab908538c 100644 (file)
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -2549,7 +2549,6 @@ PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
 {
    PREDICATELOCKTARGETTAG tag;
    ItemPointer tid;
-   TransactionId targetxmin;
 
    if (!SerializationNeededForRead(relation, snapshot))
        return;
@@ -2559,24 +2558,9 @@ PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
     */
    if (relation->rd_index == NULL)
    {
-       TransactionId myxid;
-
-       targetxmin = HeapTupleHeaderGetXmin(tuple->t_data);
-
-       myxid = GetTopTransactionIdIfAny();
-       if (TransactionIdIsValid(myxid))
-       {
-           if (TransactionIdFollowsOrEquals(targetxmin, TransactionXmin))
-           {
-               TransactionId xid = SubTransGetTopmostTransaction(targetxmin);
-
-               if (TransactionIdEquals(xid, myxid))
-               {
-                   /* We wrote it; we already have a write lock. */
-                   return;
-               }
-           }
-       }
+       /* If we wrote it; we already have a write lock. */
+       if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmin(tuple->t_data)))
+           return;
    }
 
    /*