Alerting: Add migration to clean up rule versions table (#102484)

* add migration to clean up rule versions
* drop index right before creating a new one.
* fetch only rules which version greater than toKeep

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

@@ -2,6 +2,8 @@ package ualert
 
 import (
 	"fmt"
+	"os"
+	"strconv"
 	"strings"
 
 	"github.com/google/uuid"
@@ -29,10 +31,13 @@ func AddAlertRuleGuidMigration(mg *migrator.Migrator) {
 		Nullable: false,
 		Default:  "''",
 	}))
-	mg.AddMigration("drop index in alert_rule_version table on rule_org_id, rule_uid and version columns", migrator.NewDropIndexMigration(alertRuleVersion, alertRuleVersionUDX_OrgIdRuleUIDVersion))
+
+	mg.AddMigration("cleanup alert_rule_version table", &cleanUpRuleVersionsMigration{})
 
 	mg.AddMigration("populate rule guid in alert rule table", &setRuleGuidMigration{})
 
+	mg.AddMigration("drop index in alert_rule_version table on rule_org_id, rule_uid and version columns", migrator.NewDropIndexMigration(alertRuleVersion, alertRuleVersionUDX_OrgIdRuleUIDVersion))
+
 	mg.AddMigration("add index in alert_rule_version table on rule_org_id, rule_uid, rule_guid and version columns",
 		migrator.NewAddIndexMigration(alertRuleVersion,
 			&migrator.Index{Cols: []string{"rule_org_id", "rule_uid", "rule_guid", "version"}, Type: migrator.UniqueIndex},
@@ -118,3 +123,80 @@ func (c setRuleGuidMigration) Exec(sess *xorm.Session, mg *migrator.Migrator) er
 	}
 	return nil
 }
+
+type cleanUpRuleVersionsMigration struct {
+	migrator.MigrationBase
+}
+
+var _ migrator.CodeMigration = (*cleanUpRuleVersionsMigration)(nil)
+
+func (c cleanUpRuleVersionsMigration) SQL(migrator.Dialect) string {
+	return codeMigration
+}
+
+func getBatchSize() int {
+	const defaultBatchSize = 50
+	envvar := os.Getenv("ALERT_RULE_VERSION_CLEANUP_MIGRATION_BATCH_SIZE")
+	if envvar == "" {
+		return defaultBatchSize
+	}
+	batchSize, err := strconv.Atoi(envvar)
+	if err != nil {
+		return defaultBatchSize
+	}
+	return batchSize
+}
+
+func (c cleanUpRuleVersionsMigration) Exec(sess *xorm.Session, mg *migrator.Migrator) error {
+	var batchSize = getBatchSize()
+
+	const maxRetention = 100
+	toKeep := mg.Cfg.UnifiedAlerting.RuleVersionRecordLimit
+	if toKeep <= 0 {
+		mg.Logger.Info("Rule version record limit is not set, fallback to 100", "limit", toKeep)
+		toKeep = maxRetention
+	}
+
+	var rules []alertRule
+	err := sess.Table(alertRule{}).Select("uid, version").Where("version > ?", toKeep).Find(&rules)
+	if err != nil {
+		return err
+	}
+	mg.Logger.Debug("Got alert rule UIDs with versions greater than retention", "count", len(rules))
+	batches := len(rules) / batchSize
+	if len(rules)%batchSize != 0 {
+		batches++
+	}
+
+	mg.Logger.Info("Cleaning up table `alert_rule_version`", "batchSize", batchSize, "batches", batches, "keepVersions", toKeep)
+
+	for i := 0; i < batches; i++ {
+		end := i*batchSize + batchSize
+		if end > len(rules) {
+			end = len(rules)
+		}
+		bd := strings.Builder{}
+		for idx, r := range rules[i*batchSize : end] {
+			if idx == 0 {
+				bd.WriteString(fmt.Sprintf("SELECT '%s' as uid, %d as version", r.UID, r.Version))
+				continue
+			}
+			bd.WriteString(fmt.Sprintf(" UNION ALL SELECT '%s', %d ", r.UID, r.Version))
+		}
+		_, err = sess.Exec(fmt.Sprintf(`
+			DELETE FROM alert_rule_version
+			WHERE EXISTS (
+			    SELECT 1
+			    FROM (%s) AR
+			    WHERE AR.uid = alert_rule_version.rule_uid 
+			    AND alert_rule_version.version < AR.version - %d
+			)`, bd.String(), toKeep),
+		)
+		if err != nil {
+			return err
+		}
+
+		mg.Logger.Debug(fmt.Sprintf("Batch %d of %d processed", i+1, batches))
+	}
+	return nil
+}