[ 
http://issues.apache.org/jira/browse/LUCENE-558?page=comments#action_12376781 ] 

Chuck Williams commented on LUCENE-558:
---------------------------------------

545 is certainly more general and could handle all the cases.  I looked at it 
briefly before doing this version and was concerned by the pervasiveness of its 
changes for the simple use case of loading selected fields.  It seemed it might 
break my current code (e.g., elimination of IndexReader.document(int n) and 
Fieldable as new type of object on Documents).  I thought a version that had no 
effect other than adding the new functionality would be safer and might be 
useful to others

I just took a closer look at 545.   It seems like a good implementation.  FYI, 
I can't find the definitions of FieldSelector, SetBaseFieldSelector or 
LoadFirstFielSelector? I would suggest leaving IndexReader.document(int) -- 
there does not a appear to be a reason to take it out.  Although it is also not 
difficult to add the extra null parameter, backward compatibility is a virtue.  
Presumably it would be easy to write a field selector that takes the list of 
fields to load (loading no others) and therefore get the API I need (for the 
cost of one extra allocation).


> Selective field loading
> -----------------------
>
>          Key: LUCENE-558
>          URL: http://issues.apache.org/jira/browse/LUCENE-558
>      Project: Lucene - Java
>         Type: New Feature

>   Components: Index
>     Versions: 2.0
>  Environment: All
>     Reporter: Chuck Williams
>  Attachments: LuceneTrunk.patch
>
> Provides a new api, IndexReader.document(int doc, String[] fields).  A 
> document containing only the specified fields is created.  The other fields 
> of the document are not loaded, although unfortunately uncompressed strings 
> still have to be scanned because the length information in the index is for 
> UTF-8 encoded chars and not bytes.  This is useful for applications that need 
> quick access to a small subset of the fields.  It can be used in conjunction 
> with or for some uses instead of ParallelReader.
> This is a much smaller change for a simpler use case than Lucene-545.  No 
> existing API's are affected.
> All the tests pass and new tests are added to verify the feature.

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