Merge d5e772d into 036e3a8

expected/queries.out

@@ -78,6 +78,13 @@ INSERT INTO articles VALUES (47,  7, 'abeyance', 1772);
 INSERT INTO articles VALUES (48,  8, 'alkylic', 18610);
 INSERT INTO articles VALUES (49,  9, 'anyone', 2681);
 INSERT INTO articles VALUES (50, 10, 'anjanette', 19519);
+-- first, test zero-shard query
+SELECT COUNT(*) FROM articles WHERE author_id = 1 AND author_id = 2;
+ count 
+-------
+     0
+(1 row)
+
 -- single-shard tests
 -- test simple select for a single row
 SELECT * FROM articles WHERE author_id = 10 AND id = 50;

pg_shard.c

@@ -610,9 +610,10 @@ ExtractRangeTableEntryWalker(Node *node, List **rangeTableList)
 
 /*
  * DistributedQueryShardList prunes the shards for the table in the query based
- * on the query's restriction qualifiers, and returns this list. If the function
- * cannot find any shards for the distributed table, it errors out. In other sense,
- * the function errors out or returns a non-empty list.
+ * on the query's restriction qualifiers, and returns this list. It is possible
+ * that all shards will be pruned if a query's restrictions are unsatisfiable.
+ * In that case, this function can return an empty list; however, if the table
+ * being queried has no shards created whatsoever, this function errors out.
  */
 static List *
 DistributedQueryShardList(Query *query)
@@ -640,9 +641,6 @@ DistributedQueryShardList(Query *query)
 	prunedShardList = PruneShardList(distributedTableId, restrictClauseList,
 									 shardIntervalList);
 
-	/* shouldn't be an empty list, but assert in case something's very wrong */
-	Assert(prunedShardList != NIL);
-
 	return prunedShardList;
 }
 
@@ -1113,6 +1111,17 @@ PgShardExecutorStart(QueryDesc *queryDesc, int eflags)
 			bool topLevel = true;
 			LOCKMODE lockMode = NoLock;
 			EState *executorState = NULL;
+			bool zeroShardQuery = list_length(distributedPlan->taskList) == 0;
+
+			/* if query involves zero shards, just let it hit local table */
+			if (zeroShardQuery)
+			{
+				Plan *originalPlan = distributedPlan->originalPlan;
+				plannedStatement->planTree = originalPlan;
+
+				PgShardExecutorStart(queryDesc, eflags);
+				return;
+			}
 
 			/* disallow transactions and triggers during distributed commands */
 			PreventTransactionChain(topLevel, "distributed commands");
@@ -1778,10 +1787,7 @@ ExecuteSingleShardSelect(DistributedPlan *distributedPlan, EState *executorState
 	TupleTableSlot *tupleTableSlot = NULL;
 
 	List *taskList = distributedPlan->taskList;
-	if (list_length(taskList) != 1)
-	{
-		ereport(ERROR, (errmsg("cannot execute select over multiple shards")));
-	}
+	Assert(list_length(taskList) == 1);
 
 	task = (Task *) linitial(taskList);
 	tupleStore = tuplestore_begin_heap(false, false, work_mem);

sql/queries.sql

@@ -69,6 +69,9 @@ INSERT INTO articles VALUES (48,  8, 'alkylic', 18610);
 INSERT INTO articles VALUES (49,  9, 'anyone', 2681);
 INSERT INTO articles VALUES (50, 10, 'anjanette', 19519);
 
+-- first, test zero-shard query
+SELECT COUNT(*) FROM articles WHERE author_id = 1 AND author_id = 2;
+
 -- single-shard tests
 
 -- test simple select for a single row