Streaming XML documents has many emerging applications. However, in
this paper, we show that the restrictions imposed by data streaming is
too restrictive for processing twig queries -- the core operation for
XML query processing. Previous proposed algorithm {\tt TwigStack} is
an optimal algorithm for processing twig queries with only descendent
edges over streams of nodes. We identify the cause of the
suboptimality of the {\tt TwigStack} algorithm and show that without
some control over the order of nodes, it is not possible to develop an
optimal holistic twig join algorithm. Also the computation of the twig
queries is not memory bounded. This motivates us to study two
variations of the data streaming model -- (1) offline sorting is
allowed and the algorithm is allowed to select the correct nodes to be
streamed and (2) multiple scans is allowed. We show some lower bounds of
the two variations.