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From Matt Turner <m4tt_tur...@hotmail.com>
Subject Order of fields within a Document in Lucene 2.4+
Date Thu, 25 Jun 2009 20:32:58 GMT

The Lucene FAQ says...
 
What is the order of fields returned by Document.fields()?
* Fields are returned in the same order they were added to the document. 
(now getFields() as fields is deprecated)
 
However I think this may no longer be the case in 2.4 
 
We are indexing documents in a specific order so that we can LOAD_AND_BREAK out of our FieldSelector
as early as possible.
i.e. we have typically 50 indexed fields for a document, but when we are loading results with
.doc(), we know we only need 4 of them.
 
So, our code ensures that these are added to the index first - and once the 4th field is loaded
we break out of the selector.
 
This speeds us up by an order of magnitude.
 
 
 
However, we are finding that our field selector is processing fields in alphabetical order,
not order of addition.  This means that we'd have to rename our fields to 'aaa..' in order
to guarantee they'd be processed first.
 
 
I think, but am not sure, that this bit of code causes the problem (as spotted in http://www.mail-archive.com/java-user@lucene.apache.org/msg24105.html).
It seems to have been introduced in version 2.4 (fields are in addition order in 2.3.2)
 
DocFieldProcessorPerThread.java:

   // If we are writing vectors then we must visit
   // fields in sorted order so they are written in
   // sorted order.  TODO: we actually only need to
   // sort the subset of fields that have vectors
   // enabled; we could save [small amount of] CPU
   // here.
   quickSort(fields, 0, fieldCount-1);

 
This appears to sort fields into alphabetical order.
 
Assuming that implementing the TODO would keep them in order of addition (and just keep vectors
fields themselves sorted) - is it worth raising a JIRA to fix this ?
 
 
regards,
 
matt
 

 

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