Re: patch: function xmltable

Hi

There is some opened questions - the standard (and some other databases)
requires entering XPath expression as string literal.

I am thinking so it is too strong not necessary limit - (it enforces
dynamic query in more cases), so I allowed the expressions there.

Another questions is when these expressions should be evaluated. There are
two possibilities - once per query, once per input row. I selected "once
per input row mode" - it is simpler to implement it, and it is consistent
with other "similar" generators - see the behave and related discussion to
"array_to_string" and evaluation of separator argument. The switch to "once
per query" should not be hard - but it can be strange for users, because
some his volatile expression should be stable.

Regards

Pavel