上述类的单独一个实例具有相应类的下拉列表值的引用。
The single instance of mentioned class holds the reference for drop-down values of that class.
使用该算法避免了对同一引用实例或是位于同一数据块上不同记录的重复读取，并将对磁盘的随机读取方式转变为按列表预取的读取方式。
It can avoid reading the same reference instance and records in the same data block repeatedly. Meanwhile it can convert random reading disk into doing by table.
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