From ebe5867d1773ae49f0e436639d9777a63a7121a2 Mon Sep 17 00:00:00 2001
From: Masahiko Sawada <sawada.mshk@gmail.com>
Date: Sat, 21 Oct 2017 15:50:22 +0900
Subject: [PATCH 1/3] Keep track of local writes.

---
 src/backend/access/heap/heapam.c  |    9 +++++++++
 src/backend/access/transam/xact.c |   27 +++++++++++++++++++++++++++
 src/include/access/xact.h         |    4 ++++
 3 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 54f1100..0e28e1d 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -2453,6 +2453,9 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid,
 	 */
 	CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);
 
+	/* Remember to write on local node */
+	RememberTransactionDidWrite();
+
 	/* NO EREPORT(ERROR) from here till changes are logged */
 	START_CRIT_SECTION();
 
@@ -3262,6 +3265,9 @@ l1:
 	 */
 	MultiXactIdSetOldestMember();
 
+	/* Remember to write on local node */
+	RememberTransactionDidWrite();
+
 	compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(tp.t_data),
 							  tp.t_data->t_infomask, tp.t_data->t_infomask2,
 							  xid, LockTupleExclusive, true,
@@ -4167,6 +4173,9 @@ l2:
 	 */
 	CheckForSerializableConflictIn(relation, &oldtup, buffer);
 
+	/* Remember to write on local node */
+	RememberTransactionDidWrite();
+
 	/*
 	 * At this point newbuf and buffer are both pinned and locked, and newbuf
 	 * has enough space for the new tuple.  If they are the same buffer, only
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index b37510c..e795a2f 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -117,6 +117,9 @@ TransactionId *ParallelCurrentXids;
  */
 int			MyXactFlags;
 
+/* True means transaction did any writes */
+bool		TransactionDidWrite = false;
+
 /*
  *	transaction states - transaction state from server perspective
  */
@@ -2151,6 +2154,8 @@ CommitTransaction(void)
 	XactTopTransactionId = InvalidTransactionId;
 	nParallelCurrentXids = 0;
 
+	ForgetTransactionDidWrite();
+
 	/*
 	 * done with commit processing, set current transaction state back to
 	 * default
@@ -2428,6 +2433,8 @@ PrepareTransaction(void)
 	XactTopTransactionId = InvalidTransactionId;
 	nParallelCurrentXids = 0;
 
+	ForgetTransactionDidWrite();
+
 	/*
 	 * done with 1st phase commit processing, set current transaction state
 	 * back to default
@@ -2612,6 +2619,8 @@ AbortTransaction(void)
 		pgstat_report_xact_timestamp(0);
 	}
 
+	ForgetTransactionDidWrite();
+
 	/*
 	 * State remains TRANS_ABORT until CleanupTransaction().
 	 */
@@ -4442,6 +4451,24 @@ AbortOutOfAnyTransaction(void)
 }
 
 /*
+ * RememberTransactionDidWrite --- remember a transaction did any writes
+ */
+void
+RememberTransactionDidWrite(void)
+{
+	TransactionDidWrite = true;
+}
+
+/*
+ * ForgetTransactionDidWrite --- Forget a transaction did any writes
+ */
+void
+ForgetTransactionDidWrite(void)
+{
+	TransactionDidWrite = false;
+}
+
+/*
  * IsTransactionBlock --- are we within a transaction block?
  */
 bool
diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index 118b0a8..63491de 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -91,6 +91,8 @@ extern int	MyXactFlags;
  */
 #define XACT_FLAGS_ACQUIREDACCESSEXCLUSIVELOCK	(1U << 1)
 
+/* True means transaction did any writes */
+extern bool TransactionDidWrite;
 
 /*
  *	start- and end-of-transaction callbacks for dynamically loaded modules
@@ -377,6 +379,8 @@ extern void RegisterXactCallback(XactCallback callback, void *arg);
 extern void UnregisterXactCallback(XactCallback callback, void *arg);
 extern void RegisterSubXactCallback(SubXactCallback callback, void *arg);
 extern void UnregisterSubXactCallback(SubXactCallback callback, void *arg);
+extern void RememberTransactionDidWrite(void);
+extern void ForgetTransactionDidWrite(void);
 
 extern int	xactGetCommittedChildren(TransactionId **ptr);
 
-- 
1.7.1