[Xapian-tickets] [Xapian] #452: Add support for sorting documents missing a value to end

[Xapian]

#452: Add support for sorting documents missing a value to end
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 Reporter:  richard      |       Owner:  richard  
     Type:  enhancement  |      Status:  new      
 Priority:  normal       |   Milestone:  1.2.x    
Component:  Library API  |     Version:  SVN trunk
 Severity:  normal       |   Blockedby:           
 Platform:  All          |    Blocking:           
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 Currently, when performing a sort in ascending order based on a value
 slot, documents with no value stored in the slot will be placed highest
 (since "" sorts before any other value).  However, this is often not
 desirable: in particular, if a numeric value such as price has been stored
 in the slot, and sorting is in ascending price order, it is likely that
 documents without a price stored shouldn't be ranked above the cheapest
 products.

 A KeyMaker subclass can easily be written to support this, but it would be
 useful for there to be support in a standard KeyMaker (so that users of
 languages like Python could use it).  I have a patch attached which allows
 MultiValueKeyMaker to be given a "default value" which is used in place of
 the empty value for documents with no value stored.  This is slightly more
 flexible than simply sorting to the end; for example, the median price
 stored could be tracked, and used to sort documents with no price stored
 at this position, instead of simply sorting them to the end.

 For backends which store an upper bound on the value in a slot, it is
 trivial to compute a value which sorts after all other values in the slot
 - just append a character to the upper bound.

 The patch is fairly simple, and contains a few testcases.  Exactly as
 implemented it is probably ABI incompatible (it extends an API method with
 a new parameter with a default argument), but it could be trivially
 adjusted to use an overloaded method instead for ABI compatibility if
 added during the 1.2.x series.

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Ticket URL: <http://trac.xapian.org/ticket/452>
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