[Mulgara-general] how to order walk solutions

[Paul Gearon]

Hi Greg,

On Fri, Nov 21, 2008 at 8:52 AM, Greg Jansen <count0 at email.unc.edu> wrote:
> Below I have pasted two queries and their output, note the difference in
> solution order.  The graph is a simple hierarchy of "contains"
> predicates.  I'd like to get the solutions back in the same order they
> are walked.  Is that order lost when you remove walk variables?  Is it
> sufficient to include just one variable from the walk?
> Any recommendations are much appreciated.

TQL (and a a limited extent, SPARQL) operates on set-semantic. This
means that order has no meaning. That said, it is possible to select a
variable to order by in the results. For instance, "order by $child".

However, there is no effective way to have a query return data in the
order that it "walks" a predicate. Not only doesn't it make sense from
the perspective of set theory, it would also subvert the query
optimizer.

The only way you can see the natural order that was walked is if the
WHERE clause contains nothing but the WALK statement. This is because
the query engine will have no need to re-order the data in order to
work with it. But the result is treated as a set, so the moment some
other condition is imposed upon it, then the ordering will be changed
to whatever is most efficient (hopefully).

So the best I can offer is to suggest a programmatic solution.  You'd
have to query the data twice, using the queries you gave in your
examples below. Store the first result in a list, and then convert the
second result to a map of the "ordering" column (the same data that's
in your list) to the remaining columns.

Regards,
Paul

> TQL> select $child from <rmi://127.0.0.1/server1#ri>
> where walk(
>  $p <http://cdr.unc.edu/definitions/1.0/base-model.xml#contains>
> <info:fedora/changeme:2556>
>  and $p <http://cdr.unc.edu/definitions/1.0/base-model.xml#contains> $c)
> and $c <http://cdr.unc.edu/definitions/1.0/base-model.xml#slug> $child;
> [ "mencius" ]
> [ "confucians" ]
> [ "writings" ]
> 3 rows returned.
>
> TQL> select $child $c from <rmi://127.0.0.1/server1#ri>
> where walk(
>  $p <http://cdr.unc.edu/definitions/1.0/base-model.xml#contains>
> <info:fedora/changeme:2556>
>  and $p <http://cdr.unc.edu/definitions/1.0/base-model.xml#contains> $c)
> and $c <http://cdr.unc.edu/definitions/1.0/base-model.xml#slug> $child;
> [ "confucians", info:fedora/changeme:2554 ]
> [ "mencius", info:fedora/changeme:2555 ]
> [ "writings", info:fedora/changeme:2556 ]
> 3 rows returned.
>
>
> __
> Greg Jansen
> Carolina Digital Repository
> University of North Carolina at Chapel Hill
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