proposal - Default namespaces for XPath expressions (PostgreSQL 11)

Hi

This proposal is followup of implementation of XMLTABLE.

Lot of XML documents has assigned document namespace.

<rows xmlns="http://x.y"><row><a>10</a></row></rows>

For these XML document any search path must use schema "http://x.y". This
is not too intuitive, and from XMLTABLE usage is not too user friendly,
because the default column path (same like column name) cannot be used. A
solution of this issue is default namespace - defined in SQL/XML.

example - related to previous xml

without default namespace:
XMLTABLE(NAMESPACES('http://x.y' AS aux),
'/aux:rows/aux:row' PASSING ...
COLUMNS a int PATH 'aux:a')

with default namespace
XMLTABLE(NAMESPACES(DEFAULT 'http://x.y'),
'/rows/row' PASSING ...
COLUMNS a int);

Unfortunately the libxml2 doesn't support default namespaces in XPath
expressions. Because the libxml2 functionality is frozen, there is not big
chance for support in near future. A implementation is not too hard -
although it requires simple XPath expressions state translator.

The databases with XMLTABLE implementation supports default namespace for
XPath expressions.

The patch for initial implementation is attached.

Regards

Pavel