code: func (l *Ledis) Select(index int) (*DB, error) {
  	if index < 0 || index >= l.cfg.Databases {
  		return nil, fmt.Errorf("invalid db index %d, must in [0, %d]", index, l.cfg.Databases-1)
  	}
  
  	l.dbLock.Lock()
  	defer l.dbLock.Unlock()
  
  	db, ok := l.dbs[index]
  	if ok {
  		return db, nil
  	}
  
  	db = l.newDB(index)
  	l.dbs[index] = db
  
  	go func(db *DB) {
  		l.ttlCheckerCh <- db.ttlChecker
  	}(db)
  
  	return db, nil
  }
docs: // Select chooses a database.