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From "Grant Ingersoll (JIRA)" <>
Subject [jira] Created: (LUCENE-545) Field Selection and Lazy Field Loading
Date Sat, 15 Apr 2006 14:51:00 GMT
Field Selection and Lazy Field Loading

         Key: LUCENE-545
     Project: Lucene - Java
        Type: New Feature

  Components: Store  
    Versions: 2.0    
    Reporter: Grant Ingersoll
    Priority: Minor

The patch to come shortly implements a Field Selection and Lazy Loading mechanism for Document
loading on the IndexReader.

It introduces a FieldSelector interface that defines the accept method:
FieldSelectorResult accept(String fieldName);

(Perhaps we want to expand this to take in other parameters such as the field metadata (term
vector, etc.))

Anyone can implement a FieldSelector to define how they want to load fields for a Document.
The FieldSelectorResult can be one of four values: LOAD, LAZY_LOAD, NO_LOAD, LOAD_AND_BREAK.
The FieldsReader, as it is looping over the FieldsInfo, applies the FieldSelector to determine
what should be done with the current field.

I modeled this after the mechanism.  There are two implementations to date:
SetBasedFieldSelector and LoadFirstFieldSelector.  The former takes in two sets of field names,
one to load immed. and one to load lazily.  The latter returns LOAD_AND_BREAK on the first
field encountered.  See TestFieldsReader for examples.

It should support UTF-8 (I borrowed code from Issue 509, thanks!).  See TestFieldsReader for

I added an expert method on IndexInput  named skipChars that takes in the number of characters
to skip.  This is a compromise on changing the file format of the fields to better support
seeking.  It does some of the work of readChars, but not all of it.  It doesn't require buffer
storage and it doesn't do the bitwise operations.  It just reads in the appropriate number
of bytes and promptly ignores them.  This is useful for skipping non-binary, non-compressed
stored fields.

The biggest change is by far the introduction of the Fieldable interface (as per Doug's suggestion
from a mailing list email on Lazy Field loading from a while ago).  Field is now a Fieldable.
 All uses of Field have been changed to use Fieldable.  FieldsReader.LazyField also implements

Lazy Field loading is now implemented.  It has a major caveat (that is Documented) in that
it clones the underlying IndexInput upon lazy access to the Field value.  IT IS UNDEFINED
whether a Lazy Field can be loaded after the IndexInput parent has been closed (although,
from what I saw, it does work).  I thought about adding a reattach method, but it seems just
as easy to reload the document.  See the TestFieldsReader and DocHelper for examples.

I updated a couple of other tests to reflect the new fields that are on the DocHelper document.

All tests pass.

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