Optimize policy_updated_at migration (#2362)

- Use `TRUNCATE TABLE` rather than `DELETE FROM` for improved performance.
- Move DDL statement after truncate to avoid issues with retries (due to
  column already being created).

#2360

changes/2360-updated-at

@@ -0,0 +1 @@
+* Improve performance and reliability of Policy database migrations.

server/datastore/mysql/migrations/tables/20210927143115_AddPolicyUpdatedAtColumn.go

@@ -11,16 +11,16 @@ func init() {
 }
 
 func Up_20210927143115(tx *sql.Tx) error {
-	_, err := tx.Exec("ALTER TABLE hosts ADD COLUMN policy_updated_at timestamp NOT NULL DEFAULT '2000-01-01 00:00:00'")
-	if err != nil {
-		return errors.Wrap(err, "adding policy_updated_at column")
-	}
-
-	_, err = tx.Exec("delete from policy_membership_history")
+	_, err := tx.Exec("TRUNCATE TABLE policy_membership_history")
 	if err != nil {
 		return errors.Wrap(err, "clearing policy_membership_history")
 	}
 
+	_, err = tx.Exec("ALTER TABLE hosts ADD COLUMN policy_updated_at timestamp NOT NULL DEFAULT '2000-01-01 00:00:00'")
+	if err != nil {
+		return errors.Wrap(err, "adding policy_updated_at column")
+	}
+
 	return err
 }