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From "Joshua O'Madadhain" <>
Subject RE: GoogleQueryParser
Date Thu, 12 Sep 2002 20:36:50 GMT
This issue confused me somewhat when I first encountered it from the other
direction (looking at the inputs to, and behavior of, BooleanQuery).  

However, ultimately--as I understand it--what's going on here is that
while the AND, OR syntax suggests Boolean queries, the indexing and
searching engines use the vector model (which ranks documents by their
score on the query) rather than the Boolean model (which returns all
documents which satisfy the query).  It may be that the Boolean syntax is
more misleading than useful in some cases.

Part of the problem, too (as Peter pointed out), may be that the example
queries under discussion are not well-formed Boolean queries, so it's not
necessarily clear what their behavior *should* be.

Let's consider the example:

a b OR c

If I assume a default of AND, "a AND (b OR c)" seems plausible; so
does "(a AND b) OR c"; or perhaps "(a AND c) OR (b AND c)".

I believe that the vector model interpretation of this query may be this
(with AND default):

+a +b c

which means 

"The more of the search terms 'a', 'b', 'c' that a document has, the
higher its score will be, all other things being equal; however, if a
document doesn't have 'a', or doesn't have 'b', give it score 0
regardless of any other factors."

This is probably most similar to "(a AND c) OR (b AND c)".

The Boolean interpretation of "(a AND c) OR (b AND c)" would be

"Return all documents [in any order] that have *at least one of the
following* term sets: {'a', 'c'}; {'b', 'c'}."

which, if you think about it, gives you the same document set as the
vector model interpretation.

Personally, I avoid using the QueryParser entirely and just do my own
parsing and query construction.  Part of the reason for this is that my
code is doing term expansion and reweighting, but part of it is just that
I feel that I get more power and flexibility--and less opportunity for
ambiguity such as this--by doing my own parsing.  Your mileage may vary.  


Joshua O'Madadhain Per
  Joshua O'Madadhain: Information Scientist, Musician, Philosopher-At-Tall
 It's that moment of dawning comprehension that I live for--Bill Watterson
My opinions are too rational and insightful to be those of any organization.

On Thu, 12 Sep 2002, John Cwikla wrote:

> Actually to expand a little more after a little more digging, it
> appears that the AND/OR terms are being flattened into a list of
> + - or optional query terms that are used to remove/add results
> one after the other.
> In this sense, I think the AND, OR and NOT operators seems to have
> been an afterthought to the queryparser, since they cannot give the
> same results as +, - and optional.  AND, OR and NOT should use intersections
> and unions, while + and - is doing strict adds or rejections.
> I guess I was expecting a syntax tree, but it looks like just flattening
> of terms.
> cwikla
> -----Original Message-----
> From: John Cwikla []
> Sent: Thursday, September 12, 2002 11:53 AM
> To: 'Lucene Users List'
> Subject: RE: GoogleQueryParser
> So here is the problem, AND default operator:
> a b OR c == a AND b OR c == c OR (a AND b) == c OR (+a +b) != c b +a != c +b
> +a
> However with default OR operator:
> a AND b c == a AND b OR c == c OR (a AND b) == c OR (+a +b) == c (+a +b)
> Since AND and OR do not actually mean "required" or "optional" in a strict
> boolean sense, I claim you cannot correctly use the query parser with
> a default AND operator and get results that would be expected.
> I haven't looked more into the QueryParser yet, but in the last case with
> the
> AND operator, if at some point the internal query "switched" to OR, then
> the last item would be correct if it had parenthesis like c (+a +b)
> Or is it early and I'm missing something?
> cwikla
> -----Original Message-----
> From: Halácsy Péter []
> Sent: Wednesday, September 11, 2002 11:58 PM
> To: Lucene Users List
> Subject: RE: GoogleQueryParser
> >-----Original Message-----
> >From: Philip Chan []
> >Sent: Wednesday, September 11, 2002 11:04 PM
> >To: Lucene Users List
> >Subject: RE: GoogleQueryParser
> >
> >
> >I think there's a bug, if I set the default operator to be OR, 
> >when I run
> >
> >java org.apache.lucene.queryParser.QueryParser "a AND b OR c"
> >
> >it will give me the result of "+a +b c"
> >if I set the default operator to be AND, and run it with the 
> >term "a b OR
> >c", it will give me "+a b c", which is different
> To be exact it's not a bug, it's feature ;) Well, the structured query
> language of Lucene (and Google and others) is not a strict boolean language.
> For example I think the QueryParser of Lucene do not support parenthesis: a
> AND (b OR C) 
> Instead of strict boolean logic it supports constraint on query terms: a
> query term is either required or optional or prohibited. If you write + sign
> before the term it will be required. If you write - it will be prohibited.
> The question is: is a term required or optional if you do not specify
> anything? 
> DEFAULT_OPERATOR_OR (default QueryParser):
> A B C --> all three terms are optional
> DEFAULT_OPERATOR_AND (Google style):
> A B C --> +A +B +C all three terms are required.
> Because a b OR c query is not a strict boolean query, the query parser can
> choose how to translate it. +a +b c not too good since doesn't equal to the
> result of input query c OR a b

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