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From "W. Eliot Kimber" <el...@isogen.com>
Subject Indexing XML With Lucene: Some Initial Results
Date Sun, 14 Oct 2001 14:08:06 GMT
We have continued to test our experiment of indexing XML docs by making
one Lucene doc for each element. It seems to be working pretty well,
although we haven't tried any really large-scale tests yet (will try to
do that this coming week).

I did do some informal testing with the World Religions document set
provided by Jon Bosak of Sun Microsystems. Using the Xerces DOM
implementation, it took about 75 seconds on a 900mhz PIII laptop to
index the Book of Mormon (which is the biggest of the four works at 1.5
Meg of XML data). Searches across it were essentially instantaneous (but
the index size was small in terms of the scales Lucene can support). I
have not yet profiled the cost of things like collating hit lists by XML
document (that is, all hits with the same docid field), but that should
be purely a function of Java's speed at list iteration, not anything
Lucene does. 

I also wrote client code that takes the treeloc in a given hit and looks
up the corresponding node in the source document's DOM. This code was
very fast too (again, using the Xerces DOM implementation). I had to do
this because we aren't storing any of the XML data in the index itself
(which you could do, but seemed redundant given that the original
documents are still accessible). Given the ability to store pretty much
anything in fields, you could actually capture all of the original XML
data in the Lucene index such that the original document could be
reconsitituted with sufficient fidelity. We are not currently taking
that approach because we don't to add that complexity to the Lucene
index. But it does imply that Lucene could be used as an XML store where
the original input documents are not subsequently kept. (Of course, I
don't know if this approach would preform well enough, but it almost
certainly wouldn't perform worse than existing XML-specific storage
systems that decompose docs at the element level.) [I personally don't
like storage systems that store XML documents only as decomposed bits,
which is why we're not taking that approach in this project--we're
treating the Lucene indexes as purely transient indexes over a
separately-managed authoritative datastore. This protects us, for
example, from changes to the index rules, such as changing fields from
indexed to non-indexed or changing the rules for particular fields. It's
much easier and faster to simply re-index existing docs than to do some
sort of export/re-import process.]

I'm also starting to think about additional contextual information that
could be captured in the index to make it possible to do even more
contextual qualification at the Lucene query level. Will require more
experimentation and thought.

Again, the basic approach is very simple: for a given XML document, walk
the DOM tree, creating one Lucene doc for each element node, where each
Lucene doc has a "docid" field whose value is the same for all docs
created from the same XML document, a "tagname" field, an "ancestors"
field (the ordered list of ancestor element types for the element), a
"treeloc" field, which is the DOM tree location of the element (e.g., 0
1 0 3" for the 4th child of the first child of the second child of the
document element), and a "nodetype" field that indicates the DOM node
type that has been indexed (we also index processing instructions and
comments and could do more). We also capture any attributes as fields as
well, enabling searching on attribute values.

For the text content of the document, we are capturing only the
directly-contained content of each element and indexing that as the
"content" field. We also capture all the PCDATA content for the whole
document and index it on a separate Lucene doc with a distinct node type
(e.g., "ALL_CONTENT"). This enables phrase searching that ignores
element boundaries and can also allow for faster queries if all you care
about is whether or not a given doc has some text and not which elements
have it.

We then have a front-end that both handles preparing the queries that go
to Lucene and collating the results that come back (for example,
organizing the hits by XML document or doing additional context-based
filtering that can't be done at the Lucene level). 

Cheers,

Eliot
-- 
. . . . . . . . . . . . . . . . . . . . . . . .

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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