ARQ - Extending Query Execution

This page describes the mechanisms that can be used to extend and modify query execution within ARQ. Through these mechanisms, ARQ can be used to query different graph implementations and to provide different query evaluation and optimization strategies for particular circumstances. These mechanisms are used by TDB and SDB.

ARQ can be extended in various ways to incorporate custom code into a query. Custom filter functions and property functions provide ways to add application specific code. The free text search capabilities, using Apache Lucene, are provided via a property function. Custom filter functions and property functions should be used where possible.

Jena itself can be extended by providing a new implementation of the Graph interface. This can be used to encapsulate specific specialised storage and also for wrapping non-RDF sources to look like RDF. There is a common implementation framework provided by GraphBase so only one operation, the find method, needs to be written for a read-only data source. Basic find works well in many cases, and the whole Jena API will be able to use the extension.  For higher SPARQL performance, ARQ can be extended at the basic graph matching or algebra level.

Applications writers who extend ARQ at the query execution level should be prepared to work with the source code for ARQ for specific details and for finding code to reuse. Some example can be found in the src-examples directory in the ARQ download.

Overview of ARQ Query Processing

The sequence of actions performed by ARQ to perform a query are parsing, algebra generation, execution building, high-level optimization, low-level optimization and finally evaluation. It is not usual to modify the parsing step nor the conversion from the parse tree to the algebra form, which is a fixed algorithm defined by the SPARQL standard. Extensions can modify the algebra form by transforming it from one algebra expression to another, including introducing new operators. See also the documentation on working with the SPARQL algebra in ARQ including building algebra expressions programmatically, rather than obtaining them from a query string.


The parsing step turns a query string into a Query object. The class Query represents the abstract syntax tree (AST) for the query and provides methods to create the AST, primarily for use by the parser. The query object also provides methods to serialize the query to a string. Because this is from an AST, the string produced will be very close to the original query with the same syntactic elements, but without comments, and formatted with whitespace for readability. It is not usually the best way to build a query programmatically and the AST is not normally an extension point.

The query object can be used many times. It is not modified once created, and in particular it is not modified by query execution.

Algebra generation

ARQ generates the SPARQL algebra expression for the query. After this a number of transformations can be applied (for example, identification of property functions) but the first step is the application of the algorithm in the SPARQL specification for translating a SPARQL query string, as held in a Query object into a SPARQL algebra expression. This includes the process of removing joins involving the identity pattern (the empty graph pattern).

For example, the query:

PREFIX foaf: <>
SELECT ?name ?mbox ?nick
WHERE  { ?x foaf:name  ?name ;
            foaf:mbox  ?mbox .
         OPTIONAL { ?x  foaf:nick  ?nick }


(prefix ((foaf: <>))
  (project (?name ?mbox ?nick)
        (triple ?x foaf:name ?name)
        (triple ?x foaf:mbox ?mbox)
      (bgp (triple ?x foaf:nick ?nick)

using the SSE syntax to write out the internal data-structure for the algebra.

The online SPARQL validator at can be used to see the algebra expression for a SPARQL query. This validator is also included in Fuseki.

High-Level Optimization and Transformations

There is a collection of transformations that can be applied to the algebra, such as replacing equality filters with a more efficient graph pattern and an assignment. When extending ARQ, a query processor for a custom storage layout can choose which optimizations are appropriate and can also provide its own algebra transformations.

A transform is code that converts an algebra operation into other algebra operations. It is applied using the Transformer class:

Op op = ... ;
Transform someTransform = ... ;
op = Transformer.transform(someTransform, op) ;

The Transformer class applies the transform to each operation in the algebra expression tree. Transform itself is an interface, with one method signature for each operation type, returning a replacement for the operator instance it is called on.

One such transformation is to turn a SPARQL algebra expression involving named graphs and triples into one using quads. This transformation is performed by a call to Algebra.toQuadForm.

Transformations proceed from the bottom of the expression tree to the top. Algebra expressions are best treated as immutable so a change made in one part of the tree should result in a copy of the tree above it.  This is automated by the  TransformCopy class which is the commonly used base class for writing transforms. The other helper base class is TransformBase, which provides the identify operation (returns the node supplied) for each transform operation.

Operations can be printed out in SSE syntax. The Java toString method is overridden to provide pretty printing and the static methods in WriterOp provide output to various output objects like

Low-Level Optimization and Evaluation

The step of evaluating a query is the process of executing the algebra expression, as modified by any transformations applied, to yield a stream of pattern solutions. Low-level optimizations include choosing the order in which to evaluate basic graph patterns. These are the responsibility of the custom storage layer. Low-level optimization can be carried out dynamically as part of evaluation.

Internally, ARQ uses iterators extensively. Where possible, evaluation of an operation is achieved by feeding the stream of results from the previous stage into the evaluation. A common pattern is to take each intermediate result one at a time (use QueryIterRepeatApply to be called for each binding) , substituting the variables of pattern with those in the incoming binding, and evaluating to a query iterator of all results for this incoming row. The result can be the empty iterator (one that always returns false for hasNext). It is also common to not have to touch the incoming stream at all but merely to pass it to sub-operations.

Query Engines and Query Engine Factories

The steps from algebra generation to query evaluation are carried out when a query is executed via the QueryExecution.execSelect or other QueryExecution exec operation. It is possible to carry out storage-specific operations when the query execution is created. A query engine works in conjunction with a QueryExecution created by the QueryExecutionFactory to provide the evaluation of a query pattern. QueryExecutionBase provides all the machinery for the different result types and does not need to be modified by extensions to query execution.

ARQ provides three query engine factories; the main query engine factory, one for a reference query engine and one to remotely execute a query. SDB and TDB provide their own query engine factories which they register during sub-system initialization. Both extend the main query engine described below.

The reference query engine is a direct top-down evaluation of the expression. Its purpose is to be simple so it can be easily verified and checked then its results used to check more complicated processing in the main engine and other implementations. All arguments to each operator are fully evaluated to produce intermediate in-memory tables then a simple implementation of the operator is called to calculate the results. It does not scale and does not perform any optimizations. It is intended to be clear and simple; it is not designed to be efficient.

Query engines are chosen by referring to the registry of query engine factories.

public interface QueryEngineFactory
    public boolean accept(Query query, DatasetGraph dataset, Context context) ;
    public Plan create(Query query, DatasetGraph dataset, Binding inputBinding, Context context) ;

    public boolean accept(Op op, DatasetGraph dataset, Context context) ;
    public Plan create(Op op, DatasetGraph dataset, Binding inputBinding, Context context) ;

When the query execution factory is given a dataset and query, the query execution factory tries each registered engine factory in turn calling the accept method (for query or algebra depending on how it was presented). The registry is kept in reverse registration order - the most recently registered query engine factory is tried first. The first query engine factory to return true is chosen and no further engine factories are checked.

When a query engine factory is chosen, the create method is called to return a Plan object for the execution. The main operation of the Plan interface is to get the QueryIterator for the query.

See the example in src-examples/arq.examples.engine.MyQueryEngine.

The Main Query Engine

The main query engine can execute any query. It contains a number of basic graph pattern matching implementations including one that uses the Graph.find operation so it can work with any implementation of the Jena Graph SPI. The main query engine works with general purpose datasets but not directly with quad stores; it evaluates patterns on each graph in turn. The main query engine includes optimizations for the standard Jena implementation of in-memory graphs.

High-level optimization is performed by a sequence of transformations. This set of optimizations is evolving. A custom implementation of a query engine can reuse some or all of these transformations (see Algebra.optimize which is the set of transforms used by the main query engine).

The main query engine is a streaming engine. It evaluates expressions as the client consumes each query solution. After preparing the execution by creating the initial conditions (a partial solution of one row and no bound variables or any initial bindings of variables), the main query engine calls QC.execute which is the algorithm to execute a query. Any extension that wished to reuse some of the main query engine by providing its own OpExecutor must call this method to evaluate a sub-operation.

QC.execute finds the currently active OpExecutor factory, creates an OpExecutor object and invokes it to evaluate one algebra operation.

There are two points of extension for the main query engine:

  • Stage generators, for evaluating basic graph patterns and reusing the rest of the engine.
  • OpExecutor to execute any algebra operator specially.

The standard OpExecutor invokes the stage generator mechanism to match a basic graph pattern.

Graph matching and a custom StageGenerator

The correct point to hook into ARQ for just extending basic graph pattern matching (BGPs) is to provide a custom StageGenerator.  (To hook into filtered basic graph patterns, the extension will need to provide its own OpExecutor factory). The advantage of the StageGenerator mechanism, as compared to the more general OpExecutor described below, is that it more self-contained and requires less detail about the internal evaluation of the other SPARQL algebra operators.  This extension point corresponds to section 12.6 "Extending SPARQL Basic Graph Matching".

Below is the default code to match a BGP from OpExecutor.execute(OpBGP, QueryIterator). It merely calls fixed code in the StageBuilder class.The input is a stream of results from earlier stages. The execution must return a query iterator that is all the possible ways to match the basic graph pattern for each of the inputs in turn. Order of results does not matter. 

protected QueryIterator execute(OpBGP opBGP, QueryIterator input)
    BasicPattern pattern = opBGP.getPattern() ;
    return StageBuilder.execute(pattern, input, execCxt) ;

The StageBuilder looks for the stage generator by accessing the context for the execution:

StageGenerator stageGenerator = (StageGenerator)context.get(ARQ.stageGenerator) ;

where the context is the global context and any query execution specific additions together with various execution control elements.

A StageGenerator is an implementation of:

    public interface StageGenerator
        public QueryIterator execute(BasicPattern pattern,
                                     QueryIterator input,
                                     ExecutionContext execCxt) ;

Setting the Stage Generator

An extension stage generator can be registered on a per-query execution basis or (more usually) in the global context.

    StageBuilder.setGenerator(Context, StageGenerator)

The global context can be obtained by a call to ARQ.getContext()

    StageBuilder.setGenerator(ARQ.getContext(), myStageGenerator) ;

In order to allow an extensions to still permit other graphs to be used, stage generators are usually chained, with each new custom one passing the execution request up the chain if the request is not supported by this custom stage generator.

public class MyStageGenerator implements StageGenerator
    StageGenerator above = null ;

    public MyStageGenerator (StageGenerator original)
    { above = original ; }

    public QueryIterator execute(BasicPattern pattern, QueryIterator input, ExecutionContext execCxt)
        Graph g = execCxt.getActiveGraph() ;
        // Test to see if this is a graph we support.
        if ( ! ( g instanceof MySpecialGraphClass ) )
            // Not us - bounce up the StageGenerator chain
            return above.execute(pattern, input, execCxt) ;
        MySpecialGraphClass graph = (MySpecialGraphClass )g ;
        // Create a QueryIterator for this request

This is registered by setting the global context (StageBuilder has a convenience operation to do this):

  // Get the standard one.
  StageGenerator orig = (StageGenerator)ARQ.getContext().get(ARQ.stageGenerator) ;
  // Create a new one
  StageGenerator myStageGenerator= new MyStageGenerator(orig) ;
  // Register it
  StageBuilder.setGenerator(ARQ.getContext(), myStageGenerator) ;

Example: src-examples/arq.examples.bgpmatching.


A StageGenerator provides matching for a basic graph pattern. If an extension wishes to take responsibility for more of the evaluation then it needs to work with OpExecutor. This includes evaluation of filtered basic graph patterns.

An example query using a filter:

PREFIX  dc:   <>
PREFIX  books: <>

  { ?book  dc:title  ?title .
    FILTER regex(?title, "Paddington")

results in the algebra expression for the pattern:

    (filter (regex ?title "Paddington")
        (bgp (triple ?book dc:title ?title)

showing that the filter is being applied to the results of a basic graph pattern matching.

Note: this is not the way to provide custom filter operations.  See the documentation for application-provided filter functions.

Each step of evaluation in the main query engine is performed by a OpExecutor and a new one is created from a factory at each step.  The factory is registered in the execution context. The implementation of a specialized OpExecutor can inherit from the standard one and override only those algebra operators it wishes to deal with, including inspecting the execution and choosing to pass up to the super-class based on the details of the operation.  From the query above, only regex filters might be specially handled.

Registering an OpExecutorFactory:

OpExecutorFactory customExecutorFactory = new MyOpExecutorFactory(...) ;
QC.setFactory(ARQ.getCOntext(), customExecutorFactory) ;

QC is a point of indirection that chooses the execution process at each stage in a query so if the custom execution wishes to evaluate an algebra operation within another operation, it should call QC.execute. Be careful not to loop endlessly if the operation is itself handled by the custom evaluator. This can be done by swapping in a different OpExecutorFactory.

   // Execute an operation with a different OpExecution Factory

   // New context.
   ExecutionContext ec2 = new ExecutionContext(execCxt) ;
   ec2.setExecutor(plainFactory) ;

   QueryIterator qIter = QC.execute(op, input, ec2) ;

   private static OpExecutorFactory plainFactory =
      new OpExecutorFactory()
         public OpExecutor create(ExecutionContext execCxt)
             // The default OpExecutor of ARQ.
             return new OpExecutor(execCxt) ;
      } ;


If a custom extension provides named graphs, then it may be useful to execute the quad form of the query. This is done by writing a custom query engine and overriding QueryEngineMain.modifyOp:

  protected Op modifyOp(Op op)
     // Cope with initial bindings.
     op = Substitute.substitute(op, initialInput) ;
     // Use standard optimizations.
     op = super.modifyOp(op) ;
     // Turn into quad form.
     op = Algebra.toQuadForm(op) ;
     return op ;

The extension may need to provide its own dataset implementation so that it can detect when queries are directed to its named graph storage. TDB and SDB are examples of this.

Mixed Graph Implementation Datasets

The dataset implementation used in normal operation does not work on quads but instead can provide a dataset with a collection of graphs each from different implementation sub-systems. In-memory graphs can be mixed with database backed graphs as well as custom storage systems. Query execution proceeds per-graph so that an custom OpExecutor will need to test the graph to work with to make sure it is of the right class. The pattern in the StageGenerator extension point is an example of a design pattern in that situation.

Custom Query Engines

A custom query engine enables an extension to choose which datasets it wishes to handle. It also allows the extension to intercept query execution during the setup of the execution so it can modify the algebra expression, introduce its own algebra extensions, choose which high-level optimizations to apply and also transform to the expression into quad form. Execution can proceed with the normal algorithm or a custom OpExecutor or a custom Stage Generator or a combination of all three extension mechanism.

Only a small, skeleton custom query engine is needed to intercept the initial setup. See the example in src-examples/arq.examples.engine.MyQueryEngine.

While it is possible to replace the entire process of query evaluation, this is a substantial endeavour. QueryExecutionBase provides the machinery for result presentation (SELECT, CONSTRUCT, DESCRIBE, ASK), leaving the work of pattern evaluation to the custom query engine. QueryExecutionFactory assumes that QueryExecutionBase will be used.

Algebra Extensions

New operators can be added to the algebra using the OpExt class as the super-class of the new operator. They can be inserted into the expression to be evaluated using a custom query engine to intercept evaluation initialization.  When evaluation of a query requires the evaluation of a sub-class of OpExt, the eval method is called. SDB uses this to introduce an operator that is implemented in SQL.