Optimization in ARQ proceeds on two levels. After the query is parsed, the SPARQL algebra for the query is generated as described in the SPARQL specification. High-level optimization occurs by rewriting the algebra into new, equivalent algebra forms and introducing specialized algebra operators. During query execution, the low-level, storage-specific optimization occurs such as choosing the order of triple patterns within basic graph patterns.
The effect of high-level optimizations can
be seen using arq.qparse and the low-level runtime optimizations can
be seen by execution logging.
Algebra Transformations
The preparation for a query for execution can be investigated with the
command arq.qparse --explain --query QueryFile.rq. Different storage
systems may perform different optimizations, usually chosen from the
standard set. qparse shows the action of the memory-storage optimizer
which applies all optimizations.
Other useful arguments are:
qparse arguments
| Argument | Effect |
|---|---|
--print=query |
Print the parsed query |
--print=op |
Print the SPARQL algebra for the query. This is exactly the algebra specified by the SPARQL standard. |
--print=opt |
Print the optimized algebra for the query. |
--print=quad |
Print the quad form algebra for the query. |
--print=optquad |
Print the quad-form optimized algebra for the query. |
The argument --explain is equivalent to --print=query --print=opt
Examples:
arq.qparse --explain --query Q.rq
arq.qparse --explain 'SELECT * { ?s ?p ?o }'
Execution Logging
ARQ can log query and update execution details globally or for an individual operations. This adds another level of control on top of the logger level controls.
From command line:
arq.sparql --explain --data ... --query ...
Explanatory messages are controlled by the Explain.InfoLevel level in
the execution context.
Execution logging at level ALL can cause a significant slowdown in
query execution speeds but the order of operations logged will be
correct.
The logger used is called org.apache.jena.arq.exec. Message are sent
at level “info”. So for log4j2, the following can be set in the
log4j2.properties file:
logger.arq-exec.name = org.apache.jena.arq.exec
logger.arq-exec.level = INFO
The context setting is for key (Java constant) ARQ.symLogExec. To set
globally:
ARQ.setExecutionLogging(Explain.InfoLevel.ALL);
and it may also be set on an individual query execution using its local context.
try(QueryExecution qExec = QueryExecution.create() ... .set(ARQ.symLogExec, Explain.InfoLevel.ALL).build() ) {
ResultSet rs = qExec.execSelect() ;
...
}
On the command line:
arq.query --explain --data data file --query=queryfile
The command tdbquery takes the same –explain argument.
Logging information levels: see the logging page
To get ARQ query explanation in Fuseki logs see Fuseki logging