From 10b7c686e52a6d1bb10194ebf9331ef06f044d46 Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Wed, 2 Nov 2011 08:47:43 +0000
Subject: [PATCH] Start Hot Standby faster when initial snapshot is incomplete.
 If the initial snapshot had overflowed then we can start whenever the latest
 snapshot is empty, not overflowed or as we did already, start when the xmin
 on primary was higher than xmax of our starting snapshot, which proves we
 have full snapshot data.

Bug report by Chris Redekop
---
 src/backend/storage/ipc/procarray.c | 50 ++++++++++++++++++-----------
 1 file changed, 31 insertions(+), 19 deletions(-)

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index a8ff54037c..0284ac4082 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -475,8 +475,9 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 
 	/*
 	 * If our initial RunningTransactionsData had an overflowed snapshot then
-	 * we knew we were missing some subxids from our snapshot. We can use this
-	 * data as an initial snapshot, but we cannot yet mark it valid. We know
+	 * we knew we were missing some subxids from our snapshot. If we continue
+	 * to see overflowed snapshots then we might never be able to start up,
+	 * so we make another test to see if our snapshot is now valid. We know
 	 * that the missing subxids are equal to or earlier than nextXid. After we
 	 * initialise we continue to apply changes during recovery, so once the
 	 * oldestRunningXid is later than the nextXid from the initial snapshot we
@@ -485,21 +486,31 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 	 */
 	if (standbyState == STANDBY_SNAPSHOT_PENDING)
 	{
-		if (TransactionIdPrecedes(standbySnapshotPendingXmin,
-								  running->oldestRunningXid))
+		/*
+		 * If the snapshot isn't overflowed or if its empty we can
+		 * reset our pending state and use this snapshot instead.
+		 */
+		if (!running->subxid_overflow || running->xcnt == 0)
 		{
-			standbyState = STANDBY_SNAPSHOT_READY;
-			elog(trace_recovery(DEBUG2),
-				 "running xact data now proven complete");
-			elog(trace_recovery(DEBUG2),
-				 "recovery snapshots are now enabled");
+			standbyState = STANDBY_INITIALIZED;
 		}
 		else
-			elog(trace_recovery(DEBUG2),
-				 "recovery snapshot waiting for %u oldest active xid on standby is %u",
-				 standbySnapshotPendingXmin,
-				 running->oldestRunningXid);
-		return;
+		{
+			if (TransactionIdPrecedes(standbySnapshotPendingXmin,
+									  running->oldestRunningXid))
+			{
+				standbyState = STANDBY_SNAPSHOT_READY;
+				elog(trace_recovery(DEBUG1),
+					 "recovery snapshots are now enabled");
+			}
+			else
+				elog(trace_recovery(DEBUG1),
+					 "recovery snapshot waiting for non-overflowed snapshot or "
+					 "until oldest active xid on standby is at least %u (now %u)",
+					 standbySnapshotPendingXmin,
+					 running->oldestRunningXid);
+			return;
+		}
 	}
 
 	Assert(standbyState == STANDBY_INITIALIZED);
@@ -604,7 +615,6 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 		standbyState = STANDBY_SNAPSHOT_READY;
 
 		standbySnapshotPendingXmin = InvalidTransactionId;
-		procArray->lastOverflowedXid = InvalidTransactionId;
 	}
 
 	/*
@@ -627,13 +637,15 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 
 	LWLockRelease(ProcArrayLock);
 
-	elog(trace_recovery(DEBUG2), "running transaction data initialized");
 	KnownAssignedXidsDisplay(trace_recovery(DEBUG3));
 	if (standbyState == STANDBY_SNAPSHOT_READY)
-		elog(trace_recovery(DEBUG2), "recovery snapshots are now enabled");
+		elog(trace_recovery(DEBUG1), "recovery snapshots are now enabled");
 	else
-		ereport(LOG,
-				(errmsg("consistent state delayed because recovery snapshot incomplete")));
+		elog(trace_recovery(DEBUG1),
+			 "recovery snapshot waiting for non-overflowed snapshot or "
+			 "until oldest active xid on standby is at least %u (now %u)",
+			 standbySnapshotPendingXmin,
+			 running->oldestRunningXid);
 }
 
 /*
-- 
2.40.0