According to the SQL standard, the context of XMLTABLE's XPath
row_expression is the document node of the XML input document, not the
root node.  This becomes visible when a relative path rather than
absolute is used as row expression.  Absolute paths is what was used in
original tests and docs (and the most common form used in examples
throughout the interwebs), which explains why this wasn't noticed
before.

Other functions such as xpath() and xpath_exists() also have this
problem.  While not specified by the SQL standard, it would be pretty
odd to leave those functions to behave differently than XMLTABLE, so
change them too.  However, this is a backwards-incompatible change.

No backpatch, out of fear of breaking code depending on the original
broken behavior.

Author: Markus Winand
Reported-By: Markus Winand
Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/0684A598-002C-42A2-AE12-F024A324EAE4@winand.at