Introduce citus.single_shard_commit_protocol for if users want 1PC on writes to replicas

src/backend/distributed/executor/adaptive_executor.c

@@ -937,15 +937,37 @@ DistributedExecutionRequiresRollback(DistributedExecution *execution)
 	}
 
 	if (list_length(task->taskPlacementList) > 1)
+	{
+		if (SingleShardCommitProtocol == COMMIT_PROTOCOL_2PC)
 		{
 			/*
 			 * Adaptive executor opts to error out on queries if a placement is unhealthy,
 			 * not marking the placement itself unhealthy in the process.
-		 * Use 2PC to rollback placements before the unhealthy shard failed.
+			 * Use 2PC to rollback placements before the unhealthy replica failed.
 			 */
 			return true;
 		}
 
+		/*
+		 * Some tasks don't set replicationModel thus we only
+		 * rely on the anchorShardId, not replicationModel.
+		 *
+		 * TODO: Do we ever need replicationModel in the Task structure?
+		 * Can't we always rely on anchorShardId?
+		 */
+		if (task->anchorShardId != INVALID_SHARD_ID && ReferenceTableShardId(
+				task->anchorShardId))
+		{
+			return true;
+		}
+
+		/*
+		 * Single DML/DDL tasks with replicated tables (non-reference)
+		 * should not require BEGIN/COMMIT/ROLLBACK.
+		 */
+		return false;
+	}
+
 	return false;
 }
 

src/backend/distributed/shared_library_init.c

@@ -128,7 +128,7 @@ static const struct config_enum_entry use_secondary_nodes_options[] = {
 	{ NULL, 0, false }
 };
 
-static const struct config_enum_entry multi_shard_commit_protocol_options[] = {
+static const struct config_enum_entry shard_commit_protocol_options[] = {
 	{ "1pc", COMMIT_PROTOCOL_1PC, false },
 	{ "2pc", COMMIT_PROTOCOL_2PC, false },
 	{ NULL, 0, false }
@@ -930,7 +930,22 @@ RegisterCitusConfigVariables(void)
 					 "performance benefits."),
 		&MultiShardCommitProtocol,
 		COMMIT_PROTOCOL_2PC,
-		multi_shard_commit_protocol_options,
+		shard_commit_protocol_options,
+		PGC_USERSET,
+		0,
+		NULL, NULL, NULL);
+
+	DefineCustomEnumVariable(
+		"citus.single_shard_commit_protocol",
+		gettext_noop(
+			"Sets the commit protocol for commands modifying a single shards with multiple replicas."),
+		gettext_noop("When a failure occurs during commands that modify multiple "
+					 "replicas, two-phase commit is required to ensure data is never lost "
+					 "and this is the default. However, changing to 1pc may give small "
+					 "performance benefits."),
+		&SingleShardCommitProtocol,
+		COMMIT_PROTOCOL_2PC,
+		shard_commit_protocol_options,
 		PGC_USERSET,
 		0,
 		NULL, NULL, NULL);

src/backend/distributed/transaction/transaction_management.c

@@ -38,6 +38,7 @@ CoordinatedTransactionState CurrentCoordinatedTransactionState = COORD_TRANS_NON
 
 /* GUC, the commit protocol to use for commands affecting more than one connection */
 int MultiShardCommitProtocol = COMMIT_PROTOCOL_2PC;
+int SingleShardCommitProtocol = COMMIT_PROTOCOL_2PC;
 int SavedMultiShardCommitProtocol = COMMIT_PROTOCOL_BARE;
 
 /* state needed to keep track of operations used during a transaction */

src/include/distributed/transaction_management.h

@@ -75,6 +75,7 @@ extern bool FunctionOpensTransactionBlock;
 
 /* config variable managed via guc.c */
 extern int MultiShardCommitProtocol;
+extern int SingleShardCommitProtocol;
 
 /* state needed to restore multi-shard commit protocol during VACUUM/ANALYZE */
 extern int SavedMultiShardCommitProtocol;