Optimize memory access in GetRunningTransactionData()

e85662d made GetRunningTransactionData() calculate the oldest running
transaction id within the current database.  This commit optimized this
calculation by performing a cheap transaction id comparison before fetching
the process database id, while the latter could cause extra cache misses.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240630231816.bf.nmisch%40google.com

Author: akorotkov

src/backend/storage/ipc/procarray.c

@@ -2753,8 +2753,6 @@ GetRunningTransactionData(void)
 	 */
 	for (index = 0; index < arrayP->numProcs; index++)
 	{
-		int			pgprocno = arrayP->pgprocnos[index];
-		PGPROC	   *proc = &allProcs[pgprocno];
 		TransactionId xid;
 
 		/* Fetch xid just once - see GetNewTransactionId */
@@ -2776,11 +2774,18 @@ GetRunningTransactionData(void)
 			oldestRunningXid = xid;
 
 		/*
-		 * Also, update the oldest running xid within the current database.
+		 * Also, update the oldest running xid within the current database. As
+		 * fetching pgprocno and PGPROC could cause cache misses, we do cheap
+		 * TransactionId comparison first.
 		 */
-		if (proc->databaseId == MyDatabaseId &&
-			TransactionIdPrecedes(xid, oldestDatabaseRunningXid))
-			oldestDatabaseRunningXid = xid;
+		if (TransactionIdPrecedes(xid, oldestDatabaseRunningXid))
+		{
+			int			pgprocno = arrayP->pgprocnos[index];
+			PGPROC	   *proc = &allProcs[pgprocno];
+
+			if (proc->databaseId == MyDatabaseId)
+				oldestDatabaseRunningXid = xid;
+		}
 
 		if (ProcGlobal->subxidStates[index].overflowed)
 			suboverflowed = true;