Configuring SPARQL Optimisers

Optimisers come in two forms both of which can be configured using the Configuration API:

• IQueryOptimiser are optimisers which optimise Graph Patterns in a Query
• IAlgebraOptimiser are optimisers which optimise SPARQL Algebra

Configuring Query Optimisers

Basic Configuration

Basic Configuration looks like the following:

@prefix dnr: <http://www.dotnetrdf.org/configuration#> .

_:optimiser a dnr:QueryOptimiser ;
  dnr:type "VDS.RDF.Query.Optimisation.DefaultOptimiser" .

The above configures the default optimiser which is used when no other optimiser is configured.

No Reorder Optimiser

The NoReorderOptimiser is an optimiser that only places FILTER clauses but does not otherwise reorder Graph Patterns. It can be configured like so:

@prefix dnr: <http://www.dotnetrdf.org/configuration#> .

_:optimiser a dnr:QueryOptimiser ;
  dnr:type "VDS.RDF.Query.Optimisation.NoReorderOptimiser" .

Weighted Optimiser

The WeightedOptimiser reorders Graph Patterns based on weighting calculated from statistics about data. These statistics must be invented/computed and then linked as a Graph to the optimiser like so:

@prefix dnr: <http://www.dotnetrdf.org/configuration#> .

_:optimiser a dnr:QueryOptimiser ;
  dnr:type "VDS.RDF.Query.Optimisation.WeightedOptimiser" ;
  dnr:usingGraph _:stats .

_:stats a dnr:Graph ;
  dnr:type "VDS.RDF.Graph" ;
  dnr:fromFile "stats.ttl" .

Configuring Algebra Optimisers

You can configure any IAlgebraOptimiser that has a public unparameterized constructor like so:

@prefix dnr: <http://www.dotnetrdf.org/configuration#> .

_:optimiser a dnr:AlgebraOptimiser ;
  dnr:type "VDS.RDF.Query.Optimisation.AskBgpOptimiser" .

You should only need to configure Algebra optimisers if using a custom optimiser. All the built-in optimisers are automatically registered and used where relevant.