Replace recursive call with case-by-case calls

The recursive call looked kinda scary even though it could only be made
one level deep. This just unrolls what it does explicitly. I moved some
logic into a separate function as we had hit three repetitions.

pg_shard.c

@@ -113,6 +113,7 @@ static DistributedPlan * BuildDistributedPlan(Query *query, List *shardIntervalL
 /* executor functions forward declarations */
 static void PgShardExecutorStart(QueryDesc *queryDesc, int eflags);
 static bool IsPgShardPlan(PlannedStmt *plannedStmt);
+static void NextExecutorStartHook(QueryDesc *queryDesc, int eflags);
 static LOCKMODE CommutativityRuleToLockMode(CmdType commandType);
 static void AcquireExecutorShardLocks(List *taskList, LOCKMODE lockMode);
 static int CompareTasksByShardId(const void *leftElement, const void *rightElement);
@@ -1104,24 +1105,22 @@ PgShardExecutorStart(QueryDesc *queryDesc, int eflags)
 	if (pgShardExecution)
 	{
 		DistributedPlan *distributedPlan = (DistributedPlan *) plannedStatement->planTree;
-
 		bool selectFromMultipleShards = distributedPlan->selectFromMultipleShards;
-		if (!selectFromMultipleShards)
+		bool zeroShardQuery = (list_length(distributedPlan->taskList) == 0);
+
+		if (zeroShardQuery)
+		{
+			/* if zero shards are involved, let query hit local table */
+			Plan *originalPlan = distributedPlan->originalPlan;
+			plannedStatement->planTree = originalPlan;
+
+			NextExecutorStartHook(queryDesc, eflags);
+		}
+		else if (!selectFromMultipleShards)
 		{
 			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");
@@ -1182,28 +1181,12 @@ PgShardExecutorStart(QueryDesc *queryDesc, int eflags)
 			originalPlan = distributedPlan->originalPlan;
 			plannedStatement->planTree = originalPlan;
 
-			/* call into the standard executor start, or hook if set */
-			if (PreviousExecutorStartHook != NULL)
-			{
-				PreviousExecutorStartHook(queryDesc, eflags);
-			}
-			else
-			{
-				standard_ExecutorStart(queryDesc, eflags);
-			}
+			NextExecutorStartHook(queryDesc, eflags);
 		}
 	}
 	else
 	{
-		/* call the next hook in the chain or the standard one, if no other hook was set */
-		if (PreviousExecutorStartHook != NULL)
-		{
-			PreviousExecutorStartHook(queryDesc, eflags);
-		}
-		else
-		{
-			standard_ExecutorStart(queryDesc, eflags);
-		}
+		NextExecutorStartHook(queryDesc, eflags);
 	}
 }
 
@@ -1223,6 +1206,26 @@ IsPgShardPlan(PlannedStmt *plannedStmt)
 }
 
 
+/*
+ * NextExecutorStartHook simply encapsulates the common logic of calling the
+ * next executor start hook in the chain or the standard executor start hook
+ * if no other hooks are present.
+ */
+static void
+NextExecutorStartHook(QueryDesc *queryDesc, int eflags)
+{
+	/* call into the standard executor start, or hook if set */
+	if (PreviousExecutorStartHook != NULL)
+	{
+		PreviousExecutorStartHook(queryDesc, eflags);
+	}
+	else
+	{
+		standard_ExecutorStart(queryDesc, eflags);
+	}
+}
+
+
 /*
  * CommutativityRuleToLockMode determines the commutativity rule for the given
  * command and returns the appropriate lock mode to enforce that rule. The