I've come across some unexpected (to me!) behaviour of the TDB Optimizer and
wondering if someone could shed any light on it.
For our database, (around 30 million triples, 350-odd different predicates,
around 50 named graphs, using UnionDefaultGraph - everything is in a named
graph), we've found that including the stats.opt file makes some queries
significantly slower than having no optimizer.
Some relatively complex queries run quite quickly and probably a bit quicker
with optimization than without. But in other cases, quite simple queries run
a lot slower - maybe 10 or 20 times slower with stats.opt in place than they do
without it.
Is this known behaviour?
Here's an example:
SELECT ?key ?label WHERE { <a-specific-uri> ?p ?key . ?key
<http://www.w3.org/2000/01/rdf-schema#label> ?label }
This query took around 30 seconds with stats.opt in place, and less than 2
seconds without it. (Some of that 2 seconds would have been HTTP transfer and
web page rendering time).
We're currently running TDB 0.8.9 and Joseki 3.4.3 on 64 bit Ubuntu. (Though
I've found similar behaviour on 32-bit Ubuntu with slightly older versions of
TDB and Joseki).
Thanks!
Bill
