optimize the flush function by deleting alerts based on their id (#1054)

pkg/database/alerts.go

@@ -850,34 +850,35 @@ func (c *Client) FlushAlerts(MaxAge string, MaxItems int) error {
 		deletedByAge = nbDeleted
 	}
 	if MaxItems > 0 {
-		if totalAlerts > MaxItems {
-			nbToDelete := totalAlerts - MaxItems
-			batchSize := 500
-			if batchSize > nbToDelete {
-				batchSize = nbToDelete
-			}
-			deleted := 0
-			for deleted < nbToDelete {
-				c.Log.Debugf("FlushAlerts (before query with filter) to delete: %d", nbToDelete)
-				alerts, err := c.QueryAlertWithFilter(map[string][]string{
-					"sort":  {"ASC"},
-					"limit": {strconv.Itoa(batchSize)},
-				}) // we want to delete older alerts if we reach the max number of items
-				if err != nil {
-					c.Log.Warningf("FlushAlerts (max items query) : %s", err)
-					return errors.Wrap(err, "unable to get all alerts")
-				}
-				deletedAlerts, err := c.DeleteAlertGraphBatch(alerts)
+		//We get the highest id for the alerts
+		//We substract MaxItems to avoid deleting alerts that are not old enough
+		//This gives us the oldest alert that we want to keep
+		//We then delete all the alerts with an id lower than this one
+		//We can do this because the id is auto-increment, and the database won't reuse the same id twice
+		lastAlert, err := c.QueryAlertWithFilter(map[string][]string{
+			"sort":  {"DESC"},
+			"limit": {"1"},
+		})
+		c.Log.Debugf("FlushAlerts (last alert): %+v", lastAlert)
+		if err != nil {
+			c.Log.Errorf("FlushAlerts: could not get last alert: %s", err)
+			return errors.Wrap(err, "could not get last alert")
+		}
+
+		if len(lastAlert) != 0 {
+			maxid := lastAlert[0].ID - MaxItems
+
+			c.Log.Debugf("FlushAlerts (max id): %d", maxid)
+
+			if maxid > 0 {
+				//This may lead to orphan alerts (at least on MySQL), but the next time the flush job will run, they will be deleted
+				deletedByNbItem, err = c.Ent.Alert.Delete().Where(alert.IDLT(maxid)).Exec(c.CTX)
+
 				if err != nil {
-					c.Log.Warningf("FlushAlerts (max items query) : %s", err)
-					return errors.Wrap(err, "unable to delete alerts")
-				}
-				deleted += deletedAlerts
-				if nbToDelete-deleted < batchSize {
-					batchSize = nbToDelete - deleted
+					c.Log.Errorf("FlushAlerts: Could not delete alerts : %s", err)
+					return errors.Wrap(err, "could not delete alerts")
 				}
 			}
-			deletedByNbItem = deleted
 		}
 	}
 	if deletedByNbItem > 0 {