IDToFilename does the inverse of ParseFileID.

NewFilename should be named TableFilepath -- it combines the dir with the ID to make a table filepath.

ConcatIterator 连接由多个迭代器定义的序列。 （它只适用于 TableIterators，可能只是因为它不那么通用更快。） Concat: 合并多个数组；合并多个字符串.

Iterator is an iterator for a Table.

SST 在内存中的映射 Table represents(代表) a loaded table file with the info we have about it.