diff --git i/src/backend/storage/ipc/procarray.c w/src/backend/storage/ipc/procarray.c
index 8262abd42e6..96e4a878576 100644
--- i/src/backend/storage/ipc/procarray.c
+++ w/src/backend/storage/ipc/procarray.c
@@ -1663,7 +1663,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
         TransactionId xmin;
 
         /* Fetch xid just once - see GetNewTransactionId */
-        xid = UINT32_ACCESS_ONCE(other_xids[pgprocno]);
+        xid = UINT32_ACCESS_ONCE(other_xids[index]);
         xmin = UINT32_ACCESS_ONCE(proc->xmin);
 
         /*