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From Bill Roberts <>
Subject Problems with TDB Optimizer
Date Fri, 01 Apr 2011 18:44:20 GMT
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 <>
?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).


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