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Greg Albiston
Index: trunk/content/documentation/tdb/datasets.mdtext
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--- trunk/content/documentation/tdb/datasets.mdtext (revision 1833775)
+++ trunk/content/documentation/tdb/datasets.mdtext (working copy)
@@ -51,6 +51,51 @@
...
}
+### Named Graphs & Filters
+
+Named graphs provide a convenient way to organise and store your data.
+However, be aware that in certain situations named graphs can make it
difficult for the query optimiser.
+
+For example, a query with the following structure took 29 minutes to complete:
+
+ SELECT ?b ...
+ WHERE {
+
+ GRAPH dataset:BigA {
+ ?a rdf:type my:AThing.
+ ?a noa:hasGeometry ?aData.
+ ...
+ }
+
+ GRAPH dataset:SmallB {
+ ?b rdf:type my:BThing.
+ ?b my:hasData ?bData.
+ ...
+ }
+
+ FILTER(my:filterFunction1(?bData, ?aData))
+ FILTER(my:filterFunction2(?bData, "1.0 3.0, 4.0 2.0"^^my:dataLiteral) )
+
+ }
+
+The completion duration was reduced to 7 seconds by applying the global
TDB.symUnionDefaultGraph option (see above) to the dataset and modifying the
query as follows:
+
+ SELECT ?b ...
+ WHERE {
+
+ ?a rdf:type my:AThing.
+ ?a noa:hasGeometry ?aData.
+ ...
+
+ ?b rdf:type my:BThing.
+ ?b my:hasData ?bData.
+ ...
+
+ FILTER(my:filterFunction1(?bData, ?aData))
+ FILTER(my:filterFunction2(?bData, "1.0 3.0, 4.0 2.0"^^my:dataLiteral) )
+
+ }
+
## Special Graph Names
URI | Meaning
