SDB Configuration

This page describe the configuration options available. These are options for query processing, not for the database layout and storage, which is controlled by store descriptions.


Setting Options

Options can be set globally, through out the JVM, or on a per query execution basis. SDB uses the same mechanism as ARQ.

There is a global context, which is give to each query query execution as it is created. Modifications to the global context after the query execution is created are not seen by the query execution. Modifications to the context of a single query execution do not affect any other query execution nor the global context.

A context is a set of symbol/value pairs. Symbols are used created internal to ARQ and SDB and access via Java constants. Values are any Java object, together with the values true and false, which are short for the constants of class java.lang.Boolean.

Setting globally:

 SDB.getContext().set(symbol, value) ;

Per query execution:

 QueryExecution qExec = QueryExecutionFactory.create(...) ;
 qExec.getContext.set(symbol, value) ;

Setting for a query execution happens before any query compilation or setup happens. Creation of a query execution object does not compile the query, which happens when the appropriate .exec method is called.

Current Options

Symbol Effect Default
SDB.unionDefaultGraph Query patterns on the default graph match against the union of the named graphs. false
SDB.jdbcStream Attempt to stream JDBC results. true
SDB.jdbcFetchSize Set the JDBC fetch size on the SQL query statements. Must be >= 0. unset
SDB.streamGraphAPI Stream Jena APIs (also requires jdbcStream and jdbcFetchSize false
SDB.annotateGeneratedSQL Put SQL comments in SQL true

Queries over all Named Graphs

All the named graphs can be treated as a single graph in two ways: either set the SDB option above or use the URI that refers to RDF merge of the named graphs (urn:x-arq:UnionGraph).

When querying the RDF merge of named graph, the default graph in the store is not included.

This feature applies to queries only. It does not affect the storage nor does it change loading.

To find out which named graph a triple can be found in, use GRAPH as usual.

The following special IRIs exist for use as a graph name in GRAPH only:

  • <urn:x-arq:DefaultGraph> – the default graph, even when option for named union queries is set.
  • <urn:x-arq:UnionGraph> – the union of all named graphs, even when the option for named union queries is not set.

Streaming over JDBC

By default, SDB processes results from SQL statements in a streaming fashion. It is important to close query execution objects, especially if not consuming all the results, because that causes the underlying JDBC result set to be closed. JDBC result sets can be a scarce system resource.

If this option is set, but the JDBC connection is not streaming, then this feature is harmless. Setting it false caused SDB to read all results of an SQL statement at once, treating streamed connections as unstreamed.

Note that this only streams results end-to-end if the underlying JDBC connection itself is set up to stream. Most do not in the default configuration to reduce transient resource peaks on the server under load.

Setting the fetch size enables cursors in some databases but there may be restrictions imposed by the database. See the documentation for your database for details.

In addition, operations on the graph API can be made streaming by also setting the Graph API to streaming.

Annotated SQL

SQL generation can include SQL comments to show how SPARQL has been turned into SQL. This option is true by default and always set for the command sdbprint.

SDB.getContext().setFalse(SDB.annotateGeneratedSQL) ;