Finding an entry in the auxiliary index would tell us which block to search in the main database; after searching the auxiliary index, we would have to search only that one block of the main database—at a cost of one more disk read.
在辅助索引中查找入口可以告诉我们在主数据库中要读取哪一块;查找辅助索引之后，我们只需要读取主数据库中的特定的某一个磁盘分块——通过一次磁盘读取开销。