Improving query to REPLICA_HOST_STATUS related to Aurora

The new query now handles also replicas where REPLICA_LAG_IN_MILLISECONDS == 0 .
This commit also add comments to track the changes in this query.

lib/MySQL_Monitor.cpp

@@ -4302,7 +4302,16 @@ void * monitor_AWS_Aurora_thread_HG(void *arg) {
 #ifdef TEST_AURORA
 	mmsd->async_exit_status = mysql_query_start(&mmsd->interr, mmsd->mysql, "SELECT SERVER_ID, SESSION_ID, LAST_UPDATE_TIMESTAMP, REPLICA_LAG_IN_MILLISECONDS, CPU FROM REPLICA_HOST_STATUS ORDER BY SERVER_ID");
 #else
-	mmsd->async_exit_status = mysql_query_start(&mmsd->interr, mmsd->mysql, "SELECT SERVER_ID, SESSION_ID, LAST_UPDATE_TIMESTAMP, IF(SESSION_ID = 'MASTER_SESSION_ID', 0, REPLICA_LAG_IN_MILLISECONDS) AS REPLICA_LAG_IN_MILLISECONDS, CPU FROM INFORMATION_SCHEMA.REPLICA_HOST_STATUS WHERE (REPLICA_LAG_IN_MILLISECONDS > 0 AND REPLICA_LAG_IN_MILLISECONDS <= 600000) OR SESSION_ID = 'MASTER_SESSION_ID' ORDER BY SERVER_ID");
+	// for reference we list the old queries.
+	// original implementation:
+	// mmsd->async_exit_status = mysql_query_start(&mmsd->interr, mmsd->mysql, "SELECT SERVER_ID, SESSION_ID, LAST_UPDATE_TIMESTAMP, IF(SESSION_ID = 'MASTER_SESSION_ID', 0, REPLICA_LAG_IN_MILLISECONDS) AS REPLICA_LAG_IN_MILLISECONDS, CPU FROM INFORMATION_SCHEMA.REPLICA_HOST_STATUS WHERE (REPLICA_LAG_IN_MILLISECONDS > 0 AND REPLICA_LAG_IN_MILLISECONDS <= 600000) OR SESSION_ID = 'MASTER_SESSION_ID' ORDER BY SERVER_ID");
+	// to fix a bug in Aurora , see https://github.com/sysown/proxysql/issues/3082
+	// mmsd->async_exit_status = mysql_query_start(&mmsd->interr, mmsd->mysql, "SELECT SERVER_ID, SESSION_ID, LAST_UPDATE_TIMESTAMP, IF(SESSION_ID = 'MASTER_SESSION_ID', 0, REPLICA_LAG_IN_MILLISECONDS) AS REPLICA_LAG_IN_MILLISECONDS, CPU FROM INFORMATION_SCHEMA.REPLICA_HOST_STATUS WHERE (REPLICA_LAG_IN_MILLISECONDS > 0 AND REPLICA_LAG_IN_MILLISECONDS <= 600000) OR SESSION_ID = 'MASTER_SESSION_ID' ORDER BY SERVER_ID");
+	// slightly modifying the previous query. Replacing:
+	//   "REPLICA_LAG_IN_MILLISECONDS > 0"
+	// with:
+	//   "REPLICA_LAG_IN_MILLISECONDS >= 0"
+	mmsd->async_exit_status = mysql_query_start(&mmsd->interr, mmsd->mysql, "SELECT SERVER_ID, SESSION_ID, LAST_UPDATE_TIMESTAMP, IF(SESSION_ID = 'MASTER_SESSION_ID', 0, REPLICA_LAG_IN_MILLISECONDS) AS REPLICA_LAG_IN_MILLISECONDS, CPU FROM INFORMATION_SCHEMA.REPLICA_HOST_STATUS WHERE (REPLICA_LAG_IN_MILLISECONDS >= 0 AND REPLICA_LAG_IN_MILLISECONDS <= 600000) OR SESSION_ID = 'MASTER_SESSION_ID' ORDER BY SERVER_ID");
 #endif // TEST_AURORA
 	while (mmsd->async_exit_status) {
 		mmsd->async_exit_status=wait_for_mysql(mmsd->mysql, mmsd->async_exit_status);