Alerting: Increase alert rule operation perf by replacing subquery with threshold calculation (#53069)

* Replace subquery with threshold calculation

* Use offset/limit to account for orgs with large gaps in IDs

* Collapse into one statement

* Drop dead constants

* Revert to 2 query approach

* Drop unused consts again

pkg/services/ngalert/store/alertmanager.go

@@ -22,7 +22,7 @@ var (
 	// ConfigRecordsLimit defines the limit of how many alertmanager configuration versions
 	// should be stored in the database for each organization including the current one.
 	// Has to be > 0
-	ConfigRecordsLimit int64 = 100
+	ConfigRecordsLimit int = 100
 )
 
 // GetLatestAlertmanagerConfiguration returns the lastest version of the alertmanager configuration.
@@ -206,26 +206,43 @@ func getInsertQuery(driver string) string {
 	}
 }
 
-func (st *DBstore) deleteOldConfigurations(ctx context.Context, orgID, limit int64) (int64, error) {
+func (st *DBstore) deleteOldConfigurations(ctx context.Context, orgID int64, limit int) (int64, error) {
 	if limit < 1 {
 		return 0, fmt.Errorf("failed to delete old configurations: limit is set to '%d' but needs to be > 0", limit)
 	}
-	var affactedRows int64
+
+	if limit < 1 {
+		limit = ConfigRecordsLimit
+	}
+
+	var affectedRows int64
 	err := st.SQLStore.WithDbSession(ctx, func(sess *sqlstore.DBSession) error {
+		highest := &models.AlertConfiguration{}
+		ok, err := sess.Desc("id").Where("org_id = ?", orgID).OrderBy("id").Limit(1, limit-1).Get(highest)
+		if err != nil {
+			return err
+		}
+		if !ok {
+			// No configurations exist. Nothing to clean up.
+			affectedRows = 0
+			return nil
+		}
+
+		threshold := highest.ID - 1
+		if threshold < 1 {
+			// Fewer than `limit` records even exist. Nothing to clean up.
+			affectedRows = 0
+			return nil
+		}
+
 		res, err := sess.Exec(`
 			DELETE FROM 
 				alert_configuration 
 			WHERE
 				org_id = ?
 			AND 
-				id NOT IN (
-					SELECT T.* FROM (
-						SELECT id 
-						FROM alert_configuration 
-						WHERE org_id = ? ORDER BY id DESC LIMIT ?
-					)AS T
-				)
-		`, orgID, orgID, limit)
+				id < ?
+		`, orgID, threshold)
 		if err != nil {
 			return err
 		}
@@ -233,11 +250,11 @@ func (st *DBstore) deleteOldConfigurations(ctx context.Context, orgID, limit int
 		if err != nil {
 			return err
 		}
-		affactedRows = rows
-		if affactedRows > 0 {
-			st.Logger.Info("deleted old alert_configuration(s)", "org", orgID, "limit", limit, "delete_count", affactedRows)
+		affectedRows = rows
+		if affectedRows > 0 {
+			st.Logger.Info("deleted old alert_configuration(s)", "org", orgID, "limit", limit, "delete_count", affectedRows)
 		}
 		return nil
 	})
-	return affactedRows, err
+	return affectedRows, err
 }