[Xapian-discuss] Does Xapian support retrieval optional?

[Lu Zhen]

Thanks for your reply.

But if we change the default operator to "OP_OR", it would be much slower
when facing lots of documents.

I was wondering if exists a way to label some terms of the query
"optional", so merging inverted lists would ignore these terms, but at
ranking process(cacl scores), these terms do matter.


2014-11-03 13:17 GMT+08:00 Olly Betts <olly at survex.com>:

> On Thu, Oct 30, 2014 at 11:37:15AM +0800, Lu Zhen wrote:
> > I've been using Xapian for a while. But there is a scene I don't know
> > whether supported already.
> >
> > Suppose:
> > 1. Raw query: how to make pizza
> > 2. Parsed query: how AND to AND make AND pizza
> > 3. Documents:
> >     d1: how to make pizza at home
> >     d2: 3 ways to make pizza
> >     d3: make pizza in 4 easy steps
> >
> > Question:
> > 1. During searching process, how to retrieve d2, d3 (although they don't
> > contain "how to")?
>
> Set the default operator in the QueryParser to OP_OR instead of OP_AND:
>
>
> http://xapian.org/docs/apidoc/html/classXapian_1_1QueryParser.html#a2efe48be88c4872afec4bc963f417ea5
>
> The default is actually OP_OR (for historical reasons, though this will
> probably get changed at some point), so you're presumably currently
> setting this to OP_AND explicitly.
>
> Or you could set "how" and "to" as stopwords, but that fails your
> second requirement below.
>
> > 2. Even more, how to make sure the score of d1 is higher than d2 or d3
> > (because d1 does contain "how to")?
>
> OP_OR sums the weight contributions from each term present, so this
> will generally be the case.
>
> (Strictly speaking, if you want a 100% guarantee, you'll need to pick a
> weighting scheme and parameters which will ensure that is always the
> case - I think BM25 with default parameters doesn't give this, but
> you'd probably have to create an artificial test case to see d1 not
> rank higher so I wouldn't worry about it myself.)
>
> Cheers,
>     Olly
>



-- 
谢 谢
卢 振