RE: [xml-dev] The limitations of XPath and navigation for XML database p

["Michael Kay" <mike@saxonica.com>]

> platform where there was an SQL engine and query optimizer. 
> Declarative (non-procedural) is the alternative to 
> navigational or positional logic.

It's a great mistake to equate navigational with procedural, which is the
trap this article falls into.

Navigational means finding records by virtue of their relationships with
other records, as opposed to content-based retrieval which finds records by
their content alone.

Procedural means giving a sequence of instructions to be followed in order,
typically maintaining state as you go, as opposed to declarative which
describes the output as a pure function of the input.

XPath is navigational but it is not procedural.

It's procedural logic that makes languages hard to optimize, not
navigational logic. XPath is eminently optimizable. As proof of the pudding,
different XML database engines adopt radically different strategies for
evaluating the same XPath expression.


Michael Kay
http://www.saxonica.com/