add zero shard query

src/backend/distributed/executor/multi_server_executor.c

@@ -127,6 +127,12 @@ RouterExecutablePlan(MultiPlan *multiPlan, MultiExecutorType executorType)
 	List *workerDependentTaskList = NIL;
 	bool masterQueryHasAggregates = false;
 
+	/* TODO: this is a hacky solution to allow 0 shard INSERT ... SELECT */
+	if (masterQuery == NULL)
+	{
+		return true;
+	}
+
 	/* router executor cannot execute queries that hit zero shards */
 	if (taskCount == 0)
 	{

src/backend/distributed/planner/multi_router_planner.c

@@ -261,9 +261,6 @@ CreateMultiTaskRouterPlan(Query *originalQuery, Query *query,
 		++taskIdIndex;
 	}
 
-	/* there should be a least a single task */
-	Assert(sqlTaskList != NIL);
-
 	/* Create the worker job */
 	workerJob = CitusMakeNode(Job);
 	workerJob->taskList = sqlTaskList;
@@ -360,7 +357,7 @@ CreateMultipleTaskRouterModifyTask(Query *originalQuery, Query *query,
 	}
 
 	/* Ensure that we have INSERTed table's placement exists on the same worker */
-	insertShardPlacementList = ShardPlacementList(shardId);
+	insertShardPlacementList = FinalizedShardPlacementList(shardId);
 	intersectedPlacementList = IntersectPlacementList(insertShardPlacementList,
 													  selectPlacementList);
 

src/test/regress/expected/multi_insert_select.out

@@ -223,6 +223,54 @@ DEBUG:  Plan is router executable
 DEBUG:  CommitTransactionCommand
 DEBUG:  CommitTransaction
 DEBUG:  name: unnamed; blockState:       STARTED; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children: 
+-- a zero shard select
+INSERT INTO raw_events_second (value_2, value_1, value_3, value_4, user_id, time) 
+SELECT 
+   value_2, value_1, value_3, value_4, user_id, time 
+FROM 
+   raw_events_first
+WHERE
+   false;
+DEBUG:  StartTransactionCommand
+DEBUG:  StartTransaction
+DEBUG:  name: unnamed; blockState:       DEFAULT; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children: 
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  ProcessQuery
+DEBUG:  Plan is router executable
+DEBUG:  CommitTransactionCommand
+DEBUG:  CommitTransaction
+DEBUG:  name: unnamed; blockState:       STARTED; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children: 
+-- another zero shard select
+INSERT INTO raw_events_second (value_2, value_1, value_3, value_4, user_id, time) 
+SELECT 
+   value_2, value_1, value_3, value_4, user_id, time 
+FROM 
+   raw_events_first
+WHERE
+   0 != 0;
+DEBUG:  StartTransactionCommand
+DEBUG:  StartTransaction
+DEBUG:  name: unnamed; blockState:       DEFAULT; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children: 
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  skipping the task
+DETAIL:  Insert query hits 2 placements, Select query hits 1 placements and only 1 of those placements match.
+DEBUG:  ProcessQuery
+DEBUG:  Plan is router executable
+DEBUG:  CommitTransactionCommand
+DEBUG:  CommitTransaction
+DEBUG:  name: unnamed; blockState:       STARTED; state: INPROGR, xid/subid/cid: 0/1/0, nestlvl: 1, children: 
 -- add one more row
 SET client_min_messages TO INFO;
 DEBUG:  StartTransactionCommand

src/test/regress/sql/multi_insert_select.sql

@@ -82,6 +82,24 @@ FROM
 WHERE
    user_id = 8;
 
+-- a zero shard select
+INSERT INTO raw_events_second (value_2, value_1, value_3, value_4, user_id, time) 
+SELECT 
+   value_2, value_1, value_3, value_4, user_id, time 
+FROM 
+   raw_events_first
+WHERE
+   false;
+
+
+-- another zero shard select
+INSERT INTO raw_events_second (value_2, value_1, value_3, value_4, user_id, time) 
+SELECT 
+   value_2, value_1, value_3, value_4, user_id, time 
+FROM 
+   raw_events_first
+WHERE
+   0 != 0;
 
 -- add one more row
 SET client_min_messages TO INFO;