Best practices for halyard:forkAndFilterBy

[Mark]

Any best practices/advice for how to best make use of halyard:forkAndFilterBy? Or alternatively, any practices to avoid? Are there any problems with using it more than once, e.g. inside a sub-select group-by as well as in the main where clause?

[Mark]

Here's a simple case in point:

?a rdfs:label ?l.

?b rdfs:label ?l.

Should I forkAndFilter on just ?a and ?b - for a given ?a or ?b, ?l has a low cardinality.

Or just on ?l, being the common var?

Or ?a, ?b and ?l because it is difficult to know how the query will be executed?