Alerting: Add index for rule_group_index in alert_rule table

This is a slight optimization for the list queries which sort by these fields.

pkg/services/sqlstore/migrations/migrations.go

@@ -158,4 +158,6 @@ func (oss *OSSMigrations) AddMigration(mg *Migrator) {
 	ualert.AddStateFiredAtColumn(mg)
 
 	ualert.CollateAlertRuleGroup(mg)
+
+	ualert.AddAlertRuleGroupIndexMigration(mg)
 }

pkg/services/sqlstore/migrations/ualert/alert_rule_group_index.go

@@ -0,0 +1,14 @@
+package ualert
+
+import "github.com/grafana/grafana/pkg/services/sqlstore/migrator"
+
+// AddAlertRuleGroupIndexMigration adds an index on org_id, namespace_uid, rule_group, and rule_group_idx columns to alert_rule table.
+func AddAlertRuleGroupIndexMigration(mg *migrator.Migrator) {
+	mg.AddMigration("add index in alert_rule on org_id, namespace_uid, rule_group and rule_group_idx columns", migrator.NewAddIndexMigration(
+		migrator.Table{Name: "alert_rule"},
+		&migrator.Index{
+			Name: "IDX_alert_rule_org_id_namespace_uid_rule_group_rule_group_idx",
+			Cols: []string{"org_id", "namespace_uid", "rule_group", "rule_group_idx"},
+		},
+	))
+}