Restore rule for getting global snapshot

Author: knizhnik

contrib/multimaster/multimaster.c

@@ -123,6 +123,7 @@ static TransactionId DtmNextXid;
 static SnapshotData DtmSnapshot = { HeapTupleSatisfiesMVCC };
 static bool DtmHasGlobalSnapshot;
 static int DtmLocalXidReserve;
+static CommandId DtmCurcid;
 static Snapshot DtmLastSnapshot;
 static TransactionManager DtmTM = {
 	DtmGetTransactionStatus,
@@ -630,11 +631,12 @@ static Snapshot DtmGetSnapshot(Snapshot snapshot)
 {
 	if (TransactionIdIsValid(DtmNextXid) && snapshot != &CatalogSnapshotData)
 	{
-		if (!DtmHasGlobalSnapshot) {
+		if (!DtmHasGlobalSnapshot && (snapshot != DtmLastSnapshot || DtmCurcid != GetCurrentCommandId(false))) {
 			DtmGlobalGetSnapshot(DtmNextXid, &DtmSnapshot, &dtm->minXid);
         }
 		DtmLastSnapshot = snapshot;
 		DtmMergeWithGlobalSnapshot(snapshot);
+		DtmCurcid = snapshot->curcid;
 		if (!IsolationUsesXactSnapshot())
 		{
 			/* Use single global snapshot during all transaction for repeatable read isolation level,

contrib/pg_dtm/pg_dtm.c

@@ -107,6 +107,7 @@ static SnapshotData DtmSnapshot = { HeapTupleSatisfiesMVCC };
 static bool DtmHasGlobalSnapshot;
 static bool DtmGlobalXidAssigned;
 static int DtmLocalXidReserve;
+static CommandId DtmCurcid;
 static Snapshot DtmLastSnapshot;
 static TransactionManager DtmTM = {
 	DtmGetTransactionStatus,
@@ -600,7 +601,6 @@ DtmGetNewTransactionId(bool isSubXact)
 	return xid;
 }
 
-
 static Snapshot DtmGetSnapshot(Snapshot snapshot)
 {
 	if (DtmGlobalXidAssigned)
@@ -613,11 +613,13 @@ static Snapshot DtmGetSnapshot(Snapshot snapshot)
 	}
 	if (TransactionIdIsValid(DtmNextXid) && snapshot != &CatalogSnapshotData)
 	{
+		// if (!DtmHasGlobalSnapshot && (snapshot != DtmLastSnapshot || DtmCurcid != GetCurrentCommandId(false))) {
 		if (!DtmHasGlobalSnapshot) { 
 			ArbiterGetSnapshot(DtmNextXid, &DtmSnapshot, &dtm->minXid);
 		}
 		DtmLastSnapshot = snapshot;
 		DtmMergeWithGlobalSnapshot(snapshot);
+		DtmCurcid = snapshot->curcid;
 		if (!IsolationUsesXactSnapshot())
 		{
 			/* Use single global snapshot during all transaction for repeatable read isolation level,

contrib/pg_dtm/tests/dtmbench.cpp

@@ -70,7 +70,7 @@ struct config
         nWriters = 10;
         nIterations = 1000;
         nAccounts = 1000;      
-        isolationLevel = "read committed";
+        isolationLevel = "repeatable read";//"read committed";
     }
 };