Use GetPlacementListConnection for router SELECTs

src/backend/distributed/executor/multi_router_executor.c

@@ -75,6 +75,8 @@ bool EnableDeadlockPrevention = true;
 /* functions needed during run phase */
 static void ReacquireMetadataLocks(List *taskList);
 static void AssignInsertTaskShardId(Query *jobQuery, List *taskList);
+static ShardPlacementAccess * CreatePlacementAccess(ShardPlacement *placement,
+													ShardPlacementAccessType accessType);
 static void ExecuteSingleModifyTask(CitusScanState *scanState, Task *task,
 									bool expectResults);
 static void ExecuteSingleSelectTask(CitusScanState *scanState, Task *task);
@@ -601,6 +603,7 @@ ExecuteSingleSelectTask(CitusScanState *scanState, Task *task)
 	List *taskPlacementList = task->taskPlacementList;
 	ListCell *taskPlacementCell = NULL;
 	char *queryString = task->queryString;
+	List *relationShardList = task->relationShardList;
 
 	if (XactModificationLevel == XACT_MODIFICATION_MULTI_SHARD)
 	{
@@ -620,8 +623,32 @@ ExecuteSingleSelectTask(CitusScanState *scanState, Task *task)
 		bool dontFailOnError = false;
 		int64 currentAffectedTupleCount = 0;
 		int connectionFlags = SESSION_LIFESPAN;
-		MultiConnection *connection =
-			GetPlacementConnection(connectionFlags, taskPlacement, NULL);
+		List *placementAccessList = NIL;
+		MultiConnection *connection = NULL;
+
+		if (list_length(relationShardList) > 0)
+		{
+			placementAccessList = BuildPlacementSelectList(taskPlacement->nodeName,
+														   taskPlacement->nodePort,
+														   relationShardList);
+		}
+		else
+		{
+			/*
+			 * When the SELECT prunes down to 0 shards, just use the dummy placement.
+			 *
+			 * FIXME: it would be preferable to evaluate the SELECT locally since no
+			 * data from the workers is required.
+			 */
+
+			ShardPlacementAccess *placementAccess =
+				CreatePlacementAccess(taskPlacement, PLACEMENT_ACCESS_SELECT);
+
+			placementAccessList = list_make1(placementAccess);
+		}
+
+		connection = GetPlacementListConnection(connectionFlags, placementAccessList,
+												NULL);
 
 		queryOK = SendQueryInSingleRowMode(connection, queryString, paramListInfo);
 		if (!queryOK)
@@ -641,6 +668,56 @@ ExecuteSingleSelectTask(CitusScanState *scanState, Task *task)
 }
 
 
+/*
+ * BuildPlacementSelectList builds a list of SELECT placement accesses
+ * which can be used to call StartPlacementListConnection or
+ * GetPlacementListConnection.
+ */
+List *
+BuildPlacementSelectList(char *nodeName, int nodePort, List *relationShardList)
+{
+	ListCell *relationShardCell = NULL;
+	List *placementAccessList = NIL;
+
+	foreach(relationShardCell, relationShardList)
+	{
+		RelationShard *relationShard = (RelationShard *) lfirst(relationShardCell);
+		ShardPlacement *placement = NULL;
+		ShardPlacementAccess *placementAccess = NULL;
+
+		placement = FindShardPlacementOnNode(nodeName, nodePort, relationShard->shardId);
+		if (placement == NULL)
+		{
+			ereport(ERROR, (errmsg("no active placement of shard %ld found on node "
+								   "%s:%d",
+								   relationShard->shardId, nodeName, nodePort)));
+		}
+
+		placementAccess = CreatePlacementAccess(placement, PLACEMENT_ACCESS_SELECT);
+		placementAccessList = lappend(placementAccessList, placementAccess);
+	}
+
+	return placementAccessList;
+}
+
+
+/*
+ * CreatePlacementAccess returns a new ShardPlacementAccess for the given placement
+ * and access type.
+ */
+static ShardPlacementAccess *
+CreatePlacementAccess(ShardPlacement *placement, ShardPlacementAccessType accessType)
+{
+	ShardPlacementAccess *placementAccess = NULL;
+
+	placementAccess = (ShardPlacementAccess *) palloc0(sizeof(ShardPlacementAccess));
+	placementAccess->placement = placement;
+	placementAccess->accessType = accessType;
+
+	return placementAccess;
+}
+
+
 /*
  * ExecuteSingleModifyTask executes the task on the remote node, retrieves the
  * results and stores them, if RETURNING is used, in a tuple store.

src/backend/distributed/utils/metadata_cache.c

@@ -356,6 +356,43 @@ LoadShardPlacement(uint64 shardId, uint64 placementId)
 }
 
 
+/*
+ * FindShardPlacementOnNode returns the shard placement for the given shard
+ * on the given node, or returns NULL of no placement for the shard exists
+ * on the node.
+ */
+ShardPlacement *
+FindShardPlacementOnNode(char *nodeName, int nodePort, uint64 shardId)
+{
+	ShardCacheEntry *shardEntry = NULL;
+	DistTableCacheEntry *tableEntry = NULL;
+	ShardPlacement *placementArray = NULL;
+	int numberOfPlacements = 0;
+	ShardPlacement *placementOnNode = NULL;
+	int placementIndex = 0;
+
+	shardEntry = LookupShardCacheEntry(shardId);
+	tableEntry = shardEntry->tableEntry;
+	placementArray = tableEntry->arrayOfPlacementArrays[shardEntry->shardIndex];
+	numberOfPlacements = tableEntry->arrayOfPlacementArrayLengths[shardEntry->shardIndex];
+
+	for (placementIndex = 0; placementIndex < numberOfPlacements; placementIndex++)
+	{
+		ShardPlacement *placement = &placementArray[placementIndex];
+
+		if (strncmp(nodeName, placement->nodeName, WORKER_LENGTH) == 0 &&
+			nodePort == placement->nodePort)
+		{
+			placementOnNode = CitusMakeNode(ShardPlacement);
+			CopyShardPlacement(placement, placementOnNode);
+			break;
+		}
+	}
+
+	return placementOnNode;
+}
+
+
 /*
  * ShardPlacementList returns the list of placements for the given shard from
  * the cache.

src/include/distributed/metadata_cache.h

@@ -70,6 +70,8 @@ extern bool IsDistributedTable(Oid relationId);
 extern List * DistributedTableList(void);
 extern ShardInterval * LoadShardInterval(uint64 shardId);
 extern ShardPlacement * LoadShardPlacement(uint64 shardId, uint64 placementId);
+extern ShardPlacement * FindShardPlacementOnNode(char *nodeName, int nodePort,
+												 uint64 shardId);
 extern DistTableCacheEntry * DistributedTableCacheEntry(Oid distributedRelationId);
 extern int GetLocalGroupId(void);
 extern List * DistTableOidList(void);

src/include/distributed/multi_physical_planner.h

@@ -186,7 +186,7 @@ typedef struct Task
 	char replicationModel;         /* only applies to modify tasks */
 
 	bool insertSelectQuery;
-	List *relationShardList;       /* only applies INSERT/SELECT tasks */
+	List *relationShardList;
 } Task;
 
 

src/include/distributed/multi_router_executor.h

@@ -43,5 +43,7 @@ extern TupleTableSlot * RouterMultiModifyExecScan(CustomScanState *node);
 extern int64 ExecuteModifyTasksWithoutResults(List *taskList);
 extern void ExecuteTasksSequentiallyWithoutResults(List *taskList);
 
+extern List * BuildPlacementSelectList(char *nodeName, int nodePort,
+									   List *relationShardList);
 
 #endif /* MULTI_ROUTER_EXECUTOR_H_ */