[Xapian-tickets] [Xapian] #557: Allow subqueries to use separate weighting schemes

[Xapian]

#557: Allow subqueries to use separate weighting schemes
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 Reporter:  richard      |       Owner:  olly
     Type:  enhancement  |      Status:  new 
 Priority:  normal       |   Milestone:      
Component:  Library API  |     Version:      
 Severity:  normal       |   Blockedby:      
 Platform:  All          |    Blocking:      
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 When using multiple sources of weighting information, it would be very
 handy to be able to use separate weighting schemes for some subqueries.
 This could be implemented by adding a Query operator which takes a
 subquery and a Weight object, and causes the Weight object to be used for
 all posting lists generated by the subquery.

 Example of a situation this could be useful in: Imagine a database of
 documents tagged with people IDs.  Suppose that people who are tagged in
 more events are considered more important, since they may represent
 "authorities" in the social network.  If searching for documents matching
 a query, and also matching a set of IDs, the query part may want to use
 standard BM25 weighting, but the ID part may want to use a weighting
 scheme which applies a higher weight to IDs with a higher termfrequency,
 rather than a lower weight.

 Things to think about:

  - What to do about the term-independent part of the weight (probably we'd
 just use the term-independent part of the top-level weight).

  - How does this interact with OP_SYNONYM?

  - Should the query length each weight object sees be the global query
 length, or just the length of the part of the object with the adjusted
 weight?  If the latter, should the query length of the parts of the query
 without the adjusted weight be reduced accordingly?

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Ticket URL: <http://trac.xapian.org/ticket/557>
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