Stop serializing/de-serializing router target list element

This commits prevents the target list entries for router plans
to be added in the serialized plan since they are not required.
The aim of this change is to improve performance with skipping
some of the unnecessary serializations.

src/backend/distributed/planner/multi_router_planner.c

@@ -842,6 +842,13 @@ RouterQueryJob(Query *query, Task *task)
 	job->jobQuery = query;
 	job->taskList = taskList;
 
+	/*
+	 * Router query jobs do not require jobQuery's target list. Setting it to NIL
+	 * enables us not to serialize/de-serialize while the query is passed to the
+	 * executor. This improves planning times significantly.
+	 */
+	job->jobQuery->targetList = NIL;
+
 	return job;
 }