Searches must be able on any combination of collections. A typical search includes ~ 40 collections.
Now the question is, how to implement this in lucene best.
Currently I see basically three possibilities: - create a data field containing the collection name for each document and extend the query by a or-combined list of queries on this name filed.
Are lots of different combinations of collections used frequently? Probably not. If only a handful of different subsets of collections are frequently searched, then QueryFilter could be very useful.
In this approach you construct a QueryFilter for each combination of collections, passing it the collection name query. Keep the query filter around and re-use it whenever a query with that combination of collections is made. This is very fast. It uses one bit per document per filter. So if you have a million documents and eight common combinations of collections then this would use one megabyte.
You could also keep a cache of QueryFilters in a LinkedHashMap (JDK 1.4). If the size of the cache exceeds a limit, throw away its eldest entry by overriding the removeEldestEntry method. That way, if any combination of collections is possible, but only a few are probable, you can just cache the common subsets as QueryFilters. Probably we should provide such a QueryFilterCache class with Lucene...
This is the approach that I would use.
Doug
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