code: public List<M> fetch(String... columns) {
  		if (null == columns) {
  			return this.fetch();
  		}
  		//from db
  		if (!this.syncToRedis) {
  			return this.find(SqlpKit.select(this, columns));
  		}
  		//from redis
  		return this.fetchDatasFromRedis(columns);
  	}
docs: <b>Advanced Function</b>.
  List All data. <br/>
  1. The data from DB just contains `columns` and primary keys; <br/>
  2. The data from Redis will contain all columns. <br/>
  3. Use any column value can do this. <br/>
  @param columns: will fetch columns