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From Christoph Boosz <christoph.bo...@googlemail.com>
Subject Re: faceted search performance
Date Mon, 12 Oct 2009 19:30:48 GMT
Hi Jake,

Thanks for your helpful explanation.
In fact, my initial solution was to traverse each document in the result
once and count the contained terms. As you mentioned, this process took a
lot of memory.
Trying to confine the memory usage with the facet approach, I was surprised
by the decline in performance.
Now I know it's nothing abnormal, at least.

Chris


2009/10/12 Jake Mannix <jake.mannix@gmail.com>

> Hey Chris,
>
> On Mon, Oct 12, 2009 at 10:30 AM, Christoph Boosz <
> christoph.boosz@googlemail.com> wrote:
>
> > Thanks for your reply.
> > Yes, it's likely that many terms occur in few documents.
> >
> > If I understand you right, I should do the following:
> > -Write a HitCollector that simply increments a counter
> > -Get the filter for the user query once: new CachingWrapperFilter(new
> > QueryWrapperFilter(userQuery));
> > -Create a TermQuery for each term
> > -Perform the search and read the counter of the HitCollector
> >
> > I did that, but it didn't get faster. Any ideas why?
> >
>
> This killer is the "TermQuery for each term" part - this is huge. You need
> to invert this process,
> and use your query as is, but while walking in the HitCollector, on each
> doc
> which matches
> your query, increment counters for each of the terms in that document
> (which
> means you need
> an in-memory forward lookup for your documents, like a multivalued
> FieldCache - and if you've
> got roughly the same number of terms as documents, this cache is likely to
> be as large as
> your entire index - a pretty hefty RAM cost).
>
> But a good thing to keep in mind is that doing this kind of faceting
> (massively multivalued
> on a huge term-set) requires a lot of computation, even if you have all the
> proper structures
> living in memory:
>
> For each document you look at (which matches your query), you need to look
> at all
> of the terms in that document, and increment a counter for that term.  So
> however much
> time it would normally take for you to do the driving query, it can take as
> much as that
> multiplied by the average number of terms in a document in your index.  If
> your documents
> are big, this could be a pretty huge latency penalty.
>
>  -jake
>

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