What is the subReader used for in Lucene?

I don't know what the ReaderUtil.subReader of Lucene does. Does anybody know what it does?

S开发者_如何学编程ee the class definition here: ReaderUtil.

Is it used to read each segment separately?

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A Lucene index is divided into segments. In short, from each segment only a chunk of the index is read. And subreaders are the actual readers which work directly on a segment (one segment => one segment reader). And the IndexReader, which clients use, is an aggregated implementation that uses the subreaders to perform the actual work.