3 files changed, 69 insertions, 5 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 60ea3f7f1ed..f3a8dae6312 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7599,6 +7599,16 @@ CreateCheckPoint(int flags)
 	checkPoint.time = (pg_time_t) time(NULL);
 
 	/*
+	 * For Hot Standby, derive the oldestActiveXid before we fix the redo pointer.
+	 * This allows us to begin accumulating changes to assemble our starting
+	 * snapshot of locks and transactions.
+	 */
+	if (!shutdown && XLogStandbyInfoActive())
+		checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
+	else
+		checkPoint.oldestActiveXid = InvalidTransactionId;
+
+	/*
 	 * We must hold WALInsertLock while examining insert state to determine
 	 * the checkpoint REDO pointer.
 	 */
@@ -7784,9 +7794,7 @@ CreateCheckPoint(int flags)
 	 * Update checkPoint.nextXid since we have a later value
 	 */
 	if (!shutdown && XLogStandbyInfoActive())
-		LogStandbySnapshot(&checkPoint.oldestActiveXid, &checkPoint.nextXid);
-	else
-		checkPoint.oldestActiveXid = InvalidTransactionId;
+		LogStandbySnapshot(&checkPoint.nextXid);
 
 	START_CRIT_SECTION();
 
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 878b64715c7..00dc9c853c9 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1529,6 +1529,63 @@ GetRunningTransactionData(void)
 }
 
 /*
+ * GetOldestActiveTransactionId()
+ *
+ * Similar to GetSnapshotData but returns just oldestActiveXid. We include
+ * all PGPROCs with an assigned TransactionId, even VACUUM processes.
+ * We look at all databases, though there is no need to include WALSender
+ * since this has no effect on hot standby conflicts.
+ *
+ * This is never executed during recovery so there is no need to look at
+ * KnownAssignedXids.
+ *
+ * We don't worry about updating other counters, we want to keep this as
+ * simple as possible and leave GetSnapshotData() as the primary code for
+ * that bookkeeping.
+ */
+TransactionId
+GetOldestActiveTransactionId(void)
+{
+	ProcArrayStruct *arrayP = procArray;
+	TransactionId oldestRunningXid;
+	int			index;
+
+	Assert(!RecoveryInProgress());
+
+	LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+	oldestRunningXid = ShmemVariableCache->nextXid;
+
+	/*
+	 * Spin over procArray collecting all xids and subxids.
+	 */
+	for (index = 0; index < arrayP->numProcs; index++)
+	{
+		volatile PGPROC *proc = arrayP->procs[index];
+		TransactionId xid;
+
+		/* Fetch xid just once - see GetNewTransactionId */
+		xid = proc->xid;
+
+		if (!TransactionIdIsNormal(xid))
+			continue;
+
+		if (TransactionIdPrecedes(xid, oldestRunningXid))
+			oldestRunningXid = xid;
+
+		/*
+		 * Top-level XID of a transaction is always less than any of its
+		 * subxids, so we don't need to check if any of the subxids are
+		 * smaller than oldestRunningXid
+		 */
+	}
+
+	LWLockRelease(ProcArrayLock);
+
+	return oldestRunningXid;
+}
+
+/*
  * GetTransactionsInCommit -- Get the XIDs of transactions that are committing
  *
  * Constructs an array of XIDs of transactions that are currently in commit
diff --git a/src/backend/storage/ipc/standby.c b/src/backend/storage/ipc/standby.c
index bf92d259950..f39062204c0 100644
--- a/src/backend/storage/ipc/standby.c
+++ b/src/backend/storage/ipc/standby.c
@@ -814,7 +814,7 @@ standby_desc(StringInfo buf, uint8 xl_info, char *rec)
  * making WAL entries.
  */
 void
-LogStandbySnapshot(TransactionId *oldestActiveXid, TransactionId *nextXid)
+LogStandbySnapshot(TransactionId *nextXid)
 {
 	RunningTransactions running;
 	xl_standby_lock *locks;
@@ -844,7 +844,6 @@ LogStandbySnapshot(TransactionId *oldestActiveXid, TransactionId *nextXid)
 	/* GetRunningTransactionData() acquired XidGenLock, we must release it */
 	LWLockRelease(XidGenLock);
 
-	*oldestActiveXid = running->oldestRunningXid;
 	*nextXid = running->nextXid;
 }