Assume i first use keyword search to get a DocIDSet from inverted index, then i want to sort these docIds by some numeric field, like a `updateTime`, does Lucene do this without need of loading the Document objects but only with an sorted index on `updateTime`? Which i call it "Index-Only Sort Optimization" (MUST be some equal concepts in RDBMS?)
And since Lucene has a `SortField` API, what does it do the sort? I thought SortField is just a post-processing...
