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From "Erick Erickson" <erickerick...@gmail.com>
Subject Re: setPositionIncrement questions
Date Tue, 01 Apr 2008 13:19:29 GMT
See Chris's reply, but for this <<<So I will not
want to return higher PositionIncrement for each instance of a field, just
those which I'm interested in (title/headers)>>>

I think you want PerFieldAnalyzerWrapper.

Erick

On Mon, Mar 31, 2008 at 10:56 AM, Itamar Syn-Hershko <itamar@divrei-tora.com>
wrote:

>
> Well, here is the thing - I don't necessarily want to get results per
> paragraphs - which your code will do just fine for. I want to have my
> article titles and sub-headers in the main text field, after I have
> duplicated them to give the words they contain more weight. So I will not
> want to return higher PositionIncrement for each instance of a field, just
> those which I'm interested in (title/headers). Can this be done somehow
> without injecting a "magic string", as Chris called it?
> Just so I couldn't be clearer, here is pseudo-code of my case:
>
> doc.add("field", "my title", blah, blah) /// I want to create proximity
> gap
> here
> doc.add("field", "word1 word2 word3", blah, blah)
> doc.add("field", "word4 word5 word6", blah, blah)
> doc.add("field", "my sub-header", blah, blah) /// here as well
> doc.add("field", "word7 word8 word9", blah, blah)
> IndexWriter.add(doc)
>
> >>> You can simply subclass whichever one you choose and override
> getPositionIncrementGap
>
> getPositionIncrementGap is a member function of StandardAnalyzer, not
> StandardTokenizer. Since my use case is a bit different than what you
> initially thought, I think I will wait for your thoughts on this. So far I
> have concluded that I will have to perform a check at
> StandardTokenizer::next for the "magic string", and if found set the
> current
> Token there to have PositionIncrement of about 500. Please let me know if
> there is a better way to do that (ideally without magic...).
>
> You have pretty much understood my use case for position increment 0 - but
> I
> thought this is possible to do with customizing a Scorer? I haven't gotten
> that deep into Lucene myself (yet)...
> I'm not entirely sure I understand the consequences of storing more than
> one
> Term in the same position. What I understood from your explanation is that
> if I store both "b" and "c" at the same position x, Lucene will get to x
> for
> both "b" and "c", meaning this could save me query inflation, or as I
> first
> suggested, auto-apply synonyms. The only question is, I guess, are there
> any
> drawbacks for using this?
>
> Thanks.
>
> Itamar.
>
> -----Original Message-----
> From: Erick Erickson [mailto:erickerickson@gmail.com]
> Sent: Monday, March 31, 2008 4:25 PM
> To: java-user@lucene.apache.org
> Subject: Re: setPositionIncrement questions
>
> See below...
>
> On Mon, Mar 31, 2008 at 7:02 AM, Itamar Syn-Hershko <
> itamar@divrei-tora.com>
> wrote:
>
> >
> > Chris,
> >
> > Thanks for your input.
> >
> > Please let me make sure that I get this right: while iterating through
> > the words in a document, I can use my tokenizer to
> > setPositionIncrement(150) on a specific token, what would make it be
> > more distant from the previous token than it should have been. The
> > next token will already have position increment of 1 and therefore
> > will immediately follow that token, with no extra handling. If I get
> > this right, the best way to achieve that is by appending a predefined
> > string like $$$, such that will not occur accidently in my documents,
> > and have my tokenizer set the position increment as well instead of
> > just tokenizing upon it.
>
>
> Not really. Somewhere in the indexing code is something that behaves like
> this...
>
> say you have the following lines...
> doc.add("field", "word1 word2 word3", blah, blah) doc.add("field", "word4
> word5 word6", blah, blah) doc.add("field", "word7 word8 word9", blah,
> blah)
> IndexWriter.add(doc).
>
> Now say your analyzer returns 100 for getPositionIncrementGap. The words
> will have the following offsets
> word1 - 0
> word2 - 1
> word3 - 2
> word4 - 103 (perhaps 102, but you get the idea)
> word5 - 104
> word6 - 105
> word7 - 206
> word8 - 207
> word9 - 208
>
> There's no need to have any special tokens for this to occur.
>
>
> >
> >
> > >>>  Lucene will call the "getPositionIncrementGap" method on your
> > Analyzer
> > to determine how much positionIncreiment to put in between the last
> > token of the first Field and the first token of the second Field -- so
> > you could just pass each paragraph as a seperate Field instance
> >
> > This sounds good, but is risky, since I will have to concatenate my
> > paragraphs that I DO want to have proximity data in between, and if I
> > forget to, or accidently don't do that this will corrupt
> > proximity-based searches.
> > My documents can become very big as well. I guess what I was looking
> > for was a simpler way - say tell Lucene when I do doc.add(new Field)
> > to set the position increment for the last token. The "magic char
> > sequence" will do, but I was wondering if there is a way to do that
> > without ammending my Tokenizer?
> >
>
> No, you must deal with your tokenizer, but this is pretty trivial. You can
> simply subclass whichever one you choose and override
> getPositionIncrementGap.
>
> This seems no riskier that adding your special token since you have to
> deal
> with differentiating between paragraphs you *do* want to be adjacent and
> ones you *don't* in that case as well. Or am I missing something?
>
> As to size of documents, somewhere you do need to worry about exceeding a
> position of 2^31, but if that's really an issue you have other problems
> <G>.
> Although this somewhat depends upon how far you need the paragraphs to be
> apart. Are you going to allow proximity searches of 10,000,000? Or 10?
>
>
>
> >
> > >>> it means the words appear at the same position
> >
> > ... And what does this mean exactly? How can this affect standard
> > searches?
> > What I might do with this is store stems side-by-side with the
> > original word. From what I've heard so far this is NOT how you do this
> > for English texts - you rather store them in a different field, why is
> > that? I thought if you store them side-by-side you could write a
> > Scorer (or similar) that will return all relevant results for the stem
> > of a given word, boosting words with the same exact syntax more than
> others. Any ideas on that?
> >
>
> I don't really understand what you're trying to accomplish, a use case
> would
> help. So this may be totally off base....
>
> the words "in the same position" means that if you store, say, blivet and
> blort at the same position, and the next token is bonkers, then the
> following two matches will be found:
> "blivet bonkers" "blort bonkers" (these are as exact pharses). You can
> answer much of this by getting a copy of Luke and examining test indexes
> you
> build.
>
> To boost exact matches, you have to do some fancy dancing. For instance,
> you
> could store the original word with a special token (say $) at the end, and
> *also* the
> stemmed version at the same position. Then you have to mangle your queries
> to produce something like (word$^10 OR <stemmed version of word>) for each
> search term.
>
> Best
> Erick
>
>
>
> >
> > Itamar.
> >
> > -----Original Message-----
> > From: Chris Hostetter [mailto:hossman_lucene@fucit.org]
> > Sent: Sunday, March 30, 2008 8:56 AM
> > To: Lucene Users
> > Subject: Re: setPositionIncrement questions
> >
> >
> > : Breaking proximity data has been discussed several times before, and
> > : concluded that setPositionIncrement is the way to go. In regards of
> it:
> > :
> > : 1. Where should it be called exactly to create the gap properly?
> >
> > any part of your Analyzer can set the position increment on any token
> > to indicate how far after the previous token it should be.
> >
> > : 2. Is there a way to call it directly somehow while indexing (e.g.
> > after
> > : adding a new paragraph to an existing field) instead of appending
> > $$$
> > : for example after the new string I'm indexing, and having to update
> > my
> > : tokenizer and filters so they will retain the $$$ chars, indicating
> > the
> > : gap request?
> >
> > if you add multiple Fields with the same name, Lucene will call the
> > "getPositionIncrementGap" method on your Analyzer to determine how
> > much positionIncreiment to put in between the last token of the first
> > Field and the first token of the second Field -- so you could just
> > pass each paragraph as a seperate Field instance .. alternately you
> > can have a single Field instance, and your Analyzer can use whatever
> > mechanims it wants to decide to set the position incriment to
> > something high (a line break, a magic char sequence you put in the
> > string, ... whatever you want)
> >
> > : 3. What is the recommended value to pass setPositionIncrement to
> > create
> > : a reasonable gap, and not risk large documents being indexed
> > improperly
> > : (I mean, is there some sort of high-bound for the position value?).
> >
> > MAX_INT .. pick gaps based on your data and the queries you expect (if
> > you want gaps betwen paragraps, and your paragraphs tend to be under
> > 200 words long, make the gap 500 so "lucene java"~300 can find those
> > words in the same paragram, but can never span multiple paragraphs
> >
> > : 4. What are the consequences of setting PositionIncrement to 0? Does
> > : this mean I can index synonyms or stems aside of the "real" words
> > : without risking data corruption?
> >
> > it means the words appear at the same position - synonyms is a great
> > example of this use case.
> >
> >
> > -Hoss
> >
> >
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> >
> >
> >
> >
> >
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> >
> >
>
>
>
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