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Pardon a cut-and-paste error. The source for the text below was not the LegoDB
paper. It's from an evaluation of several alternative binary encoding schemes,
schema-based optimization, and skip pointers.
"The subsequent experiments reveal that the application overhead of extracting
relevant data from the XML document can be kept rather small. For the simplest
queries, the XQuery overhead was less than the parser overhead. For more
complex queries, the experiments revealed a clear shift in overhead from parsing
the XQuery application, as indicated by the decrease in speedups between expat
the binary encoding parsers."
The authors also noted:
"Our results have shown that with the exception of trivial binary encoding
strategies, most binary encoding optimizations yield performance improvements in
only limited applications or situations, and/or restrict the ability for
pipelined XML processing. This supports the contention in  that there is not
one binary encoding standard that could satisfy the needs of all applications.
On the contrary, however, a trivial binary encoding standard would appear to at
least provide performance benefits to most applications, without any significant
drawbacks other than compromising the view-source principle."
"An Evaluation of Binary XML Encoding Optimizations for Fast Stream Based XML