Field Selection and Lazy Field Loading
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Key: LUCENE-545
URL: http://issues.apache.org/jira/browse/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 java.io.FileFilter 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 examples
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 Fieldable.
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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