From 859cd9db48cdde68c7767359e7cdae5b72a3011f Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Fri, 10 Jan 2020 10:37:44 -0300
Subject: [PATCH] Use checkpoint timestamp to update apply lag tracker

Previously we only used transaction commit records, which makes the wal
pointer in standby lag behind when the primary is idle.
---
 src/backend/access/transam/xlog.c | 37 ++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 7f4f784c0e..59679b31ec 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5562,8 +5562,8 @@ exitArchiveRecovery(TimeLineID endTLI, XLogRecPtr endOfLog)
  *
  * If the record contains a timestamp, returns true, and saves the timestamp
  * in *recordXtime. If the record type has no timestamp, returns false.
- * Currently, only transaction commit/abort records and restore points contain
- * timestamps.
+ * Currently, only transaction commit/abort records, restore points and
+ * checkpoints contain timestamps.
  */
 static bool
 getRecordTimestamp(XLogReaderState *record, TimestampTz *recordXtime)
@@ -5577,6 +5577,13 @@ getRecordTimestamp(XLogReaderState *record, TimestampTz *recordXtime)
 		*recordXtime = ((xl_restore_point *) XLogRecGetData(record))->rp_time;
 		return true;
 	}
+	if (rmid == RM_XLOG_ID && (info == XLOG_CHECKPOINT_ONLINE ||
+							   info == XLOG_CHECKPOINT_SHUTDOWN))
+	{
+		*recordXtime =
+			time_t_to_timestamptz(((CheckPoint *) XLogRecGetData(record))->time);
+		return true;
+	}
 	if (rmid == RM_XACT_ID && (xact_info == XLOG_XACT_COMMIT ||
 							   xact_info == XLOG_XACT_COMMIT_PREPARED))
 	{
@@ -5745,9 +5752,6 @@ recoveryStopsBefore(XLogReaderState *record)
 
 /*
  * Same as recoveryStopsBefore, but called after applying the record.
- *
- * We also track the timestamp of the latest applied COMMIT/ABORT
- * record in XLogCtl->recoveryLastXTime.
  */
 static bool
 recoveryStopsAfter(XLogReaderState *record)
@@ -5755,7 +5759,6 @@ recoveryStopsAfter(XLogReaderState *record)
 	uint8		info;
 	uint8		xact_info;
 	uint8		rmid;
-	TimestampTz recordXtime;
 
 	/*
 	 * Ignore recovery target settings when not in archive recovery (meaning
@@ -5823,10 +5826,6 @@ recoveryStopsAfter(XLogReaderState *record)
 	{
 		TransactionId recordXid;
 
-		/* Update the last applied transaction timestamp */
-		if (getRecordTimestamp(record, &recordXtime))
-			SetLatestXTime(recordXtime);
-
 		/* Extract the XID of the committed/aborted transaction */
 		if (xact_info == XLOG_XACT_COMMIT_PREPARED)
 		{
@@ -5865,7 +5864,7 @@ recoveryStopsAfter(XLogReaderState *record)
 		{
 			recoveryStopAfter = true;
 			recoveryStopXid = recordXid;
-			recoveryStopTime = recordXtime;
+			getRecordTimestamp(record, &recoveryStopTime);
 			recoveryStopLSN = InvalidXLogRecPtr;
 			recoveryStopName[0] = '\0';
 
@@ -6049,6 +6048,19 @@ recoveryApplyDelay(XLogReaderState *record)
 	return true;
 }
 
+/*
+ * Track the timestamp of the latest applied time-bearing WAL record in
+ * XLogCtl->recoveryLastXTime.
+ */
+static void
+updateRecoveryXTime(XLogReaderState *record)
+{
+	TimestampTz	recordXtime;
+
+	if (getRecordTimestamp(record, &recordXtime))
+		SetLatestXTime(recordXtime);
+}
+
 /*
  * Save timestamp of latest processed commit/abort record.
  *
@@ -7255,6 +7267,9 @@ StartupXLOG(void)
 						WalSndWakeup();
 				}
 
+				/* update last recovery time */
+				updateRecoveryXTime(xlogreader);
+
 				/* Exit loop if we reached inclusive recovery target */
 				if (recoveryStopsAfter(xlogreader))
 				{
-- 
2.20.1