PG-197: Fix conflict with pg_stat_statements.

If pg_stat_monitor is loaded after pg_stat_statement, then it will end
up calling standard_planner function twice in the pgss_planner_hook()
function, this will trigger an assertion failure from PostgreSQL as this
function expects an untouched Query* object, and the first call to
standard_planner() done by pg_stat_statements modifies the object.

To address the problem, we avoid calling standard_planner function twice
in pg_stat_monitor, if a previous handler is installed for the hook
planner_hook, then we assume that this previous hook has already called
standard_planner function and don't do it again.

pg_stat_monitor.c

@@ -699,8 +699,16 @@ pgss_planner_hook(Query *parse, const char *query_string, int cursorOptions, Par
 		plan_nested_level++;
 		PG_TRY();
 		{
+			/*
+			 * If there is a previous installed hook, then assume it's going to call
+			 * standard_planner() function, otherwise we call the function here.
+			 * This is to avoid calling standard_planner() function twice, since it
+			 * modifies the first argument (Query *), the second call would trigger an
+			 * assertion failure.
+			 */
 			if (planner_hook_next)
 				result = planner_hook_next(parse, query_string, cursorOptions, boundParams);
+			else
 				result = standard_planner(parse, query_string, cursorOptions, boundParams);
 		}
 		PG_FINALLY();
@@ -734,8 +742,16 @@ pgss_planner_hook(Query *parse, const char *query_string, int cursorOptions, Par
 	}
 	else
 	{
+		/*
+		* If there is a previous installed hook, then assume it's going to call
+		* standard_planner() function, otherwise we call the function here.
+		* This is to avoid calling standard_planner() function twice, since it
+		* modifies the first argument (Query *), the second call would trigger an
+		* assertion failure.
+		*/
 		if (planner_hook_next)
 			result = planner_hook_next(parse, query_string, cursorOptions, boundParams);
+		else
 			result = standard_planner(parse, query_string, cursorOptions, boundParams);
 	}
 	return result;