On Jun13, 2011, at 21:24 , Peter Eisentraut wrote:
> On ons, 2011-06-08 at 10:14 +0200, Florian Pflug wrote:
>> But then you lose the ability to evaluate user-supplied
>> XPath expressions, because there's no way of telling which of these
>> function to use.
>
> Perhaps having both variants, one type-safe and one not, would work. I
> don't agree with doing away with type-safety completely for the sake of
> convenience.
In theory, I agree.
In practice, however, XPath 1.0 isn't very strongly typed itself. The
built-in types are all auto-converted into one another (As if
string(), number(), boolean() had been called). Also, only three of
the functions defined by XPath 1.0 seem interesting. Here's the break-down
The functions returning "string" are string(): Converts arbitrary values to strings local-name(): Name of node-set's
firsttop-level w/o namespace prefix namespace-uri(): Namespace of node-set's first top-level name(): Namespace prefix
andname of node-set's first top-level node concat() starts-with() contains() substring-before() substring-after()
substring()string-length() translate()
For all of these function postgres provides corresponding SQL functions,
which the exception of local-name() namespace-uri() name()
In fact, these three functions are the raison d'être for my patch and this thread.
I needed to find the name of a tag returned by an XPath expression, and to my
dismay discovered that XPATH('local-name(...)', ...) returns an empty array. The
only reason I added support for boolean and numeric XPath expressions at all was
for the sake of completeness.
Here's the rest of the functions defined by XPath 1.0. I'm convinces that none
of them are particularly useful as top-level functions, and therefore believe
that adding XPATH_BOOLEAN() and XPATH_NUMBER() is largely overkill.
The functions returning "number" are number(): Converts arbitrary values to numbers last() position() count() sum():
Sumover a node-set after implicit conversion of nodes to numbers floor() ceiling() round() operators +, -, *, div, mod
The functions returning "boolean" are boolean(): Converts arbitrary to boolean not() true() false() operators or, and,
=,!=, <=, <, >=, >
best regards,
Florian Pflug