It's worth pointing out that in functions like distinct-values, min, and max,
the collation is ignored if not required. I think that should be the behaviour
for order-by as well. (Indeed, I assumed it was; and making the change in this
direction would definitely involve less disruption for users.)

The working group discussed this bug report on 8 Jan 2008. Two alternatives
were discussed:

(1) Close the bug with no changes. In this case, specifying a collation for a
non-string ordering-key is an error. It was observed that this behavior is
inconsistent with max, min, distinct-values, and possibly a future group-by.

(2) Change XQuery Section 3.8.3 as follows:
When two orderspec values V and W are compared to determine their relative
order in the ordering sequence, the "greater-than" relationship is defined as
follows:
When the orderspec specifies "empty least", the following rules are applied in
order:
(a) If V is an empty sequence and W is not an empty sequence, then W
"greater-than" V is true.
(b) If V is NaN and W is neither NaN nor an empty sequence, then W
"greater-than" V is true.
(c) If a specific collation C is specified, and V and W are both of type
xs:string or are convertible to xs:string by subtype substitution and/or type
promotion, and fn:compare(V, W, C) is less than zero, then W "greater than" V
is true.
(d) If W gt V is true, then W "greater-than" V is true.
(e) If none of the above rules apply, then W "greater-than" V is false.
Analogous changes apply when the orderspec specifies "empty greatest".

If adopted, these changes could be made effective immediately by an erratum to
XQuery 1.0, or could be introduced in XQuery 1.1 as relaxation of an error
condition.

This bug report remains open pending discussion of these alternatives.
--Don Chamberlin (for the Query Working Group)

On 23 Jan 2008, the Query Working Group considered this bug report and decided
to accept alternative (2) in Comment #2. This will have the effect that a
specified collation is ignored if the ordering key is not convertible to the
type xs:string. This change will be published in an erratum to XQuery 1.0.
--Don Chamberlin (for the Query Working Group)

Mike Kay has pointed out that the text in Comment #2 does not accurately
reflect the intent of the working group that ordering is based on fn:compare()
when a collation is specified and the ordering keys are convertible to strings.
As stated in Comment #2, if fn:compare() returns a non-negative result (Rule
c), the algorithm falls through to Rule d and applies the "gt" operator. This
is a mistake.

The corrected rules are as follows:

When the orderspec specifies empty least, the following rules are applied in
order:
(1) If V is an empty sequence and W is not an empty sequence, then W
greater-than V is true.
(2) If V is NaN and W is neither NaN nor an empty sequence, then W greater-than
V is true.
(3) If a specific collation C is specified, and V and W are both of type
xs:string or are convertible to xs:string by subtype substitution and/or type
promotion, then:
If fn:compare(V, W, C) is less than zero, then W greater-than V is true;
otherwise W greater-than V is false.
(4) If none of the above rules apply, then:
If W gt V is true, then W greater-than V is true; otherwise W greater-than V is
false.

An analogous set of rules apply when the orderspec specifies empty greatest.

Since this change simply corrects an error in accurately reflecting the
decision of the working group, I am leaving this bug report in "Closed" status.

Don Chamberlin