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copied!<p>The FieldCache uses weak references to index readers as keys for their cache. (By calling <code>IndexReader.GetCacheKey</code> which has been un-obsoleted.) A standard call to <code>IndexReader.Open</code> with a <code>FSDirectory</code> will use a pool of readers, one for every segment.</p> <p>You should always pass the innermost reader to the FieldCache. Check out <code>ReaderUtil</code> for some helper stuff to retrieve the individual reader a document is contained within. Document ids wont change within a segment, what they mean when describing it as unpredictable/volatile is that it will change between two index commits. Deleted documents could have been proned, segments have been merged, and such actions.</p> <p>A commit needs to remove the segment from disk (merged/optimized away), which means that new readers wont have the pooled segment reader, and the garbage collection will remove it as soon as all older readers are closed.</p> <p>Never, ever, call <code>FieldCache.PurgeAllCaches()</code>. It's meant for testing, not production use.</p> <p>Added 2011-04-03; example code using subreaders.</p> <pre><code>var directory = FSDirectory.Open(new DirectoryInfo("index")); var reader = IndexReader.Open(directory, readOnly: true); var documentId = 1337; // Grab all subreaders. var subReaders = new List<IndexReader>(); ReaderUtil.GatherSubReaders(subReaders, reader); // Loop through all subreaders. While subReaderId is higher than the // maximum document id in the subreader, go to next. var subReaderId = documentId; var subReader = subReaders.First(sub => { if (sub.MaxDoc() < subReaderId) { subReaderId -= sub.MaxDoc(); return false; } return true; }); var values = FieldCache_Fields.DEFAULT.GetInts(subReader, "newsdate"); var value = values[subReaderId]; </code></pre>

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