What's the most efficient way to retrieve all matching documents from a query in Lucene, unsorted?

Question

I am looking to perform a query for the purposes of maintaining internal integrity; for example, removing all traces of a particular field/value from the index. Therefore it's important that I find all matching documents (not just the top n docs), but the order they are returned in is irrelevant.

According to the docs, it looks like I need to use the Searcher.Search( Query, Collector ) method, but there's no built in Collector class that does what I need.

Should I derive my own Collector for this purpose? What do I need to keep in mind when doing that?

Answer 1

Turns out this was a lot easier than I expected. I just used the example implementation at http://lucene.apache.org/java/2_9_0/api/core/org/apache/lucene/search/Collector.html and recorded the doc numbers passed to the Collect() method in a List, exposing this as a public Docs property.

I then simply iterate this property, passing the number back to the Searcher to get the proper Document:

var searcher = new IndexSearcher( reader );
var collector = new IntegralCollector(); // my custom Collector
searcher.Search( query, collector );
var result = new Document[ collector.Docs.Count ];
for ( int i = 0; i < collector.Docs.Count; i++ )
    result[ i ] = searcher.Doc( collector.Docs[ i ] );
searcher.Close(); // this is probably not needed
reader.Close();

So far it seems to be working fine in preliminary tests.

Update: Here's the code for IntegralCollector:

internal class IntegralCollector: Lucene.Net.Search.Collector {
    private int _docBase;

    private List<int> _docs = new List<int>();
    public List<int> Docs {
        get { return _docs; }
    }

    public override bool AcceptsDocsOutOfOrder() {
        return true;
    }

    public override void Collect( int doc ) {
        _docs.Add( _docBase + doc );
    }

    public override void SetNextReader( Lucene.Net.Index.IndexReader reader, int docBase ) {
        _docBase = docBase;
    }

    public override void SetScorer( Lucene.Net.Search.Scorer scorer ) {
    }
}