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From "W. Eliot Kimber" <el...@isogen.com>
Subject Re: Index Optimization: Which is Better?
Date Fri, 12 Oct 2001 14:20:39 GMT
"Steven J. Owens" wrote:

>      I think that's exactly what Elliot is intending.

Steven is correct. For each element in the XML document we create a
separate Lucene document with the following fields:

- docid (unique identifier of the input XML document, e.g., file system
path, object ID from a repository, URL, etc.)
- list of ancestor element types
- DOM tree location
- text of direct PCDATA content
- DOM node type (Element_node, processing_instruction_node,
comment_node) [This list is probably imcomplete but it was enough for us
to test the idea.]
- For each attribute of the element, a field whose name is the attribute
name and whose value is the attribute value.

We also capture all the text content of the input XML document as a
single Lucene document with the same docid and the node type
"all_content". 

Given these Lucene documents, I can do queries like this:

   big brown dog AND ancestor:tag2 AND NOT ancestor:tag3 and
language:english

This will result in one doc for each element instance that contains the
text "big brown dog", is within a tag2 element, not within a tag3
element and has the value "english" for its language attribute.

To make sure you match the phrase if it crosses element boundaries, just
include the all-content doc as well:

   big brown dog ((AND ancestor:tag2 AND NOT ancestor:tag3 and
language:english) OR
   (nodetype:ALL_CONTENT))

Given this set of Lucene docs, we can then collect them by docid to
determine which XML documents are represented. The ancestor list and
tree location enable correlating each hit back to its original location
in the input document. It also enables post-processing to do more
involved contextual filtering, such as "find 'foo' in all paras that are
first children of chapters".

We have implemented a first pass at code that does this indexing but we
have no idea how it will perform (we only got this fully working
yesterday and haven't had time to stress it yet).

I agree that this is somewhat "twisted". In fact my collegue John
Heintz, who suggested the approach of one Lucene doc per element,
characterized the idea as an "abuse" of Lucene's design. But we haven't
been able to think of a better or easier way to do it.

It was really easy to write the DOM processing code to generate this
index and the interaction with Lucene's API couldn't have been
easier--this is my first experience programming against Lucene and I'm
really impressed with the simplicity of the API and the power of the
architecture. 

The functionality described above for XML retrieval already surpasses
anything I know how to do with Verity, Fulcrum, Excallibur, etc. and it
was freaky easy to do once we got the idea for the approach. I just hope
it performs adequately.

Cheers,

E.

-- 
. . . . . . . . . . . . . . . . . . . . . . . .

W. Eliot Kimber | Lead Brain

1016 La Posada Dr. | Suite 240 | Austin TX  78752
    T 512.656.4139 |  F 512.419.1860 | eliot@isogen.com

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