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From "Erick Erickson" <>
Subject Re: One (large) field shared by many documents
Date Sat, 19 May 2007 23:34:12 GMT
You're right, your index will bloat considerably. In fact, I'm surprised
it's only a factor of 5....

The only thing that comes to mind is really a variant on your approach
from your first e-mail. But I wouldn't use document ids because document
IDs can change. So using doc IDs fraught.

So here's the variant. Go ahead and index your "collection vector",
but index it with a second field that is your "collection ID". Then, add
that collection ID to each document in your original index. So, you have
something like

a: text:{look, a, cat}  collectionID:32
b: text:{my, chimpansee, is , hairy} collectionID:32
c: text:{dogs, are, playful} collectionID:32

Your other index has
collectionID:32 collectionVector:{look, a, cat, my, chimpansee, is , hairy,
dogs, are, playful}

Now, you essentially make two queries, one to get a set of
collection IDs from your second index (that is, querying your terms
against collectionVector) and using that set of collectionIDs in a
query against your first index.

You might be able to do some interesting things with boosts
to score either query more to your liking.

This will come close to doubling the size of your index, but your
first approach could bloat it by an arbitrary factor depending upon
how many documents were in your largest collection.....

One thing to note, however, is that there is no need to have
two separate physical indexes. Lucene does not require that
all  documents have the same fields. So this could all be in one
big happy index. As long as the fields are different in the two
sets of documents, the queries won't interfere with each other. In
that case, you'd have to name the "foreign key" field differently for
the sets of documents, say collectionID1 and collectionID2.

All that said, this approach bothers me because it's mixing
some database ideas with a Lucene index. I suppose in a controlled
situation where you won't be trying to do arbitrary joins it's probably
a misplaced unease. But I'm leery of trying to make Lucene act
like a database. But that may just be a personal problem <G>

The only other consideration is "how many collections do you have?"
The reason I ask is that in the worst case scenario, you'll have an
OR clause for every collection ID you have. Lucene can easily handle
many thousands of terms in an OR, but your search time will suffer.
And you'll have to take special action (really, just set MaxBooleanClauses)
if this is over 1024 or you'll get a TooManyClauses exception.


On 5/19/07, Peter Bloem <> wrote:
> I'm sorry, I should have explained the intended behavior more clearly.
> The basic idea (without the collection fields) is that there are very
> simple documents in the index with one content field each. All I do with
> this index is a standard search in this text field. To improve the
> search results, I want to also add the concatenation of all documents in
> a collection as a field to every single document. I then search the
> index using both fields, and diminishing the effect of the collection
> field. This should improve the search results.
> As an example, say I have the documents a:"look a cat" b:"my chimpansee
> is hairy" c:"dogs are playful" and many others. These three documents
> are grouped into one collection (of many). The term vectors for the
> documents would then be
> a: {look, a, cat}
> b: {my, chimpansee, is , hairy}
> c: {dogs, are, playful}
> If I create a term vector for the whole collection: {look, a, cat, my,
> chimpansee, is , hairy, dogs, are, playful} and add it to each of the
> documents as a separate field, the query "my hairy cat" scores well
> against document a because of the match on cat, but also because of the
> match on both cat and hairy on the collection field. Documents about the
> linux command 'cat' do not have the word "hairy" in their collection
> field (because they're part of a different collection), and so would not
> get this benefit. It's essentially a smoothing technique, since it
> allows query words that aren't in the document to still have some effect.
> The problem of course is that storing these collection term vectors for
> each document greatly increases the size of the index and the indexing
> time. It would be alot faster if I could somehow use a second index to
> store the collections as documents, so I would only have to store one
> term vector per collection. (This isn't my own idea btw, I'm trying to
> replicate the results from some other research that used this method).
> I hope this is more clear,
> Peter
> Erick Erickson wrote:
> > This seems kind of kludgy, but that may just mean I don't understand
> > your problem very well.
> >
> > What is it that you're trying to accomplish? Searching constrained
> > by topic or groups?
> >
> > If you're trying to search by groups, search the archive for the
> > word "facet" or "faceted search".
> >
> > Otherwise, could you describe what behavior you're after and maybe
> > there'd be more ideas....
> >
> > Best
> > Erick
> >
> > On 5/19/07, Peter Bloem <> wrote:
> >>
> >> Hi,
> >>
> >> I have the following problem. I'm indexing documents that belong to
> some
> >> collection (ie. the dataset is divided into collections, which are
> >> divided into documents). These documents become my lucene documents,
> >> with some relatively small string that becomes the field I want to
> >> search. However, I would also like to add to document d the
> >> concatenation of all documents in d's collection as a field (mainly as
> a
> >> smoothing technique, because documents correspond roughly to topics).
> >> I'm currently doing just that, adding an extra field for the entire
> >> concatenated collection to each document in that collection. Of course
> >> this increases the index size and indexing time greatly (about
> >> five-fold).
> >>
> >> There must be a better way to do this. My idea was to create a second
> >> index where the collections are indexed as (lucene) documents. This
> >> index would have the text as a field, and a list of document id's
> >> referring back to the main index. I could then retrieve the term vector
> >> for each collection from this second index for each search result from
> >> the original index.
> >>
> >> My question is if this is a smart approach. And if it is, which of
> >> Lucene's classes should I use for this. The best I could find was the
> >> FilterIndexReader. If extending the FilterIndexReader is really the
> best
> >> way to go, could I simply override the document(int, FieldSelector)
> >> method, or is there more to it? I doubt I'm the first person that's
> ever
> >> wanted a many to one relation between fields and documents, so I hope
> >> there's a simpler way about this.
> >>
> >> Thank you,
> >> Peter
> >>
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