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From Paul Elschot <paul.elsc...@xs4all.nl>
Subject Re: Faceted search with OpenBitSet/SortedVIntList
Date Sat, 07 Feb 2009 22:06:41 GMT
On Saturday 07 February 2009 19:57:19 Raffaella Ventaglio wrote:
> Hi,
> 
> I am trying to implement a kind of faceted search using Lucene 2.4.0.
> 
> I have a list of configuration rules that tell me how to generate this
> facets and the corresponding queries (that can range from simple term
> queries to complex boolean queries).
> 
> When my application starts, it creates the whole set of facets objects and
> initializes them.
> For each facet:
> - I create the query according to the configured rule;
> - I ask the reader for the bitset corresponding to that query and I store it
> in the Facet object;
> - I get the cardinality of the bitset and I save it in the Facet object as
> its "initial count".
> 
> When the user does a search I have to update the "counts" associated to each
> Facet:
> - I get the bitset corresponding to the "query + filter" generated by the
> user search;
> - I get the cardinality of the ("search bitset" AND "facet bitset") and I
> save it as the updated count.
> 
> 
> In my first solution, I used only "OpenBitSetDISI" objects, both for Facet
> bitset and for search bitset.
> So I could use "intersectionCount" method to get updated counts after user
> search.
> 
> This works very well and it is very fast, but when the number of documents
> in the index and the number of facets grows it is too memory consuming.
> 
> 
> So I tried a different solution: when I create facet bitsets I use the same
> rule applied in ChainedFilter/BooleanFilter to decide if I have to store an
> OpenBitSet or a SortedVIntList.
> When I have to calculate updated counts:
> - if the facet has an OpenBitSet, I use the "intersectionCount" method
> directly;
> - if the facet has a SortedVIntList, I first create a new OpenBitSetDISI
> using the SortedVIntList.iterator and then I use the "intersectionCount"
> method.
> 
> In this way, I use a smaller amount of memory at initialization time, but
> for each user search I create a large number of objects (that I suddenly
> throw away) and this affects application performance because it wastes a lot
> of time doing GC.
> 
> So my question is: is there a better way to accomplish this task?
> 
> I think, it would be fine if I could calculate "intersectionCount" directly
> on SortedVIntList objects, but I have not found nothing like that in Lucene
> 2.4 JavaDoc.
> Am I missing something?

You are not missing anything.

OpenBitSet has an optimized implementation for intersection count, and
there is no counterpart of that in SortedVIntList because until now there
has been no need for it.

One way to implement that would be to use one of the boolean combination
filters in contrib, BooleanFilter or ChainedFilter,  and simply count the
the number of times next() returns true on the result.

In case the performance of that is not good enough, another way would be
to directly add an intersection count method to SortedVIntList.
However, SortedVIntList does not allow for an efficient iterator
implementation of skipTo(), and skipTo() is used intensively by intersections.
 
> As a reference, now my index contains more than 500.000 documents and I have
> to create/manage up to 50.000 facets.
> Using "second solution", at initialization time my facets structure requires
> more or less 120MB (and this is good enough), while updating counts it uses
> even 2GB of memory (and this is very bad).

50.000 facets? Well, in case the performance of the last suggestion is
not good enough, one could try and implement a better data structure
than OpenBitSet and SortedVIntList to provide a DocIdSetIterator,
preferably with a fast skipTo() and possibly with a fast intersection count.
In that case, you may want to ask further on the java-dev list.

Regards,
Paul Elschot

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