On 03/10/13 14:08, Guy Laden wrote:
We are considering porting an application that makes use of Sesame to
Fuseki/TDB and it looks like to do so we'd need to enable
unionDefaultGraph.
We would like to understand whether enabling unionDefaultGraph (with TDB)
would have an effect on performance? e.g. additional index lookups/updates?
Thanks, Guy
How much data do you have?
Accessing the names graph to form the unionDefaultGraph is done at full
speed. It's a bit fo CPU-processing on top of index access.
In TDB, there are indexes for a triple table and a quad table. There
are no additional indexes for the unionDefaultGraph, which only exists
at query time.
unionDefaultGraph is achieved by accessing a quad index when a triple
index woudl otherwise be used. So to access xyz (e.g. SPO) index, the
code goes to xyzG and removes adjacent duplicates (so it appears as a a
set of triples). Adjacent duplicate removal is very cheap (there was a
fairly recent imporvement in this area to switch to adjacent removal).
The costs should be minimal; the indexes aren't as compact so if you
were to store all data in one named graph and access as the default
graph, it may be a bit slower; you may not be able to notice until you
have a lot of data.
Overall, the effect should be zero or small.
Andy
