On Feb 20, 2012, at 7:12 PM, Andy Seaborne wrote:
>> Otherwise it inserts a
>> ?this a ?THIS_TYPE clause into the start of the WHERE clause to
>> optimize performance.
>
> What happens if ?this is used in several places?
It should work fine - it's bound in the beginning of the block so anything
"under" that will iterate over the same instances.
> If you process the algebra, then the overhead is less as you can process the
> post-optimizer algebra and execute that directly.
Yes, but my understanding of this part of Jena is not good enough yet to go
into such a low level. The other problem is simply the number of different
cases to consider. For example take
WHERE {
GRAPH ex:other {
BIND (ex:getLabel(?this) AS ?label) .
}
}
Assuming the labels are in a different graph than the rdf:type triples (that
inform us about the instances) then this pre-processing becomes very hard. I
cannot simply say
WHERE {
GRAPH ex:other {
?this a ?TYPE_CLASS .
BIND (ex:getLabel(?this) AS ?label) .
}
}
As soon as exceptions from the rule such as the one above show up, I believe I
need some outer looping mechanism anyway. I could optimize it from there for
special cases.
> Do you need to have this split mode execution? Why not build a single query
> that calculates ?this (we do this in the linked data API), not puts it in a
> repeated substitution?
How would this look like exactly?
Thanks
Holger
