[Bug 5023] Relationship between identity constraints and assertions

http://www.w3.org/Bugs/Public/show_bug.cgi?id=5023





--- Comment #12 from Noah Mendelsohn <noah_mendelsohn@us.ibm.com>  2009-04-15 15:32:27 ---
Michael Kay writes:

> There are many constraints that people want to
> express that require the full power of XPath. I
> don't regard it as a "concern" that people are
> able to express constraints that are expensive to
> evaluate - it's their decision.

Fair enough, but that doesn't quite capture my concern. I worry some that,
since XPath in general doesn't stream without a lot of hard work, many
implementors might not do an efficient job even with the simple XPaths that do.
 So, instead of the situation you describe, in which a smart user knows which
paths will stream and which won't, even the simple ones will run much slower
than they might.

That said:  it's certainly the case that nobody would be dumb enough to use
space that's the square of the document size just to evaluate @a > @b, even
though doing so might be tempting for:

test="every $a in //empno, $b in //empno satisfies $a is $b or $a ne $b"

So I think your point holds to a significant degree anyway.  It will be
unfortunate if implementations don't take the trouble to notice and optimize
the simple common cases.

Noah


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Received on Wednesday, 15 April 2009 15:32:39 UTC