WITH MALFORMED XPATH EXP
Problem:
A malformed XPATH expression in the ExtractValue query is 
causing a server crash. This malformed XPATH expression is
resulted when the position attribute in the substring function
contains ".." in the beginning.
Solution:
The original crash is happening because the "../" is being 
evaluated prematurely. It tries to access XML while it 
hasn't been parsed yet. The premature evaluation is happening
because the val_nodeset function is being set to constant, 
in which case we proceed to evaluate them in JOIN:prepare
stage only. The solution to this is setting the val_nodeset
functions as non-constant. This forces us to evaluate the function
 in the JOIN:exec stage and thus avoid any premature evaluation of
the XML strings.