SPARQL Standards Compliance in Amazon Neptune - Amazon Neptune

SPARQL Standards Compliance in Amazon Neptune

Amazon Neptune complies with the following standards in implementing the SPARQL graph query language.

Applicable Standards for SPARQL

Neptune SPARQL Implementation Specifics

The following sections provide specific details of how SPARQL is implemented in Neptune.

Default Namespace Prefixes in Neptune SPARQL

Neptune defines the following prefixes by default for use in SPARQL queries. For more information, see Prefixed Names in the SPARQL specification.

  • rdf  – http://www.w3.org/1999/02/22-rdf-syntax-ns#

  • rdfs – http://www.w3.org/2000/01/rdf-schema#

  • owl  – http://www.w3.org/2002/07/owl#

  • xsd  – http://www.w3.org/2001/XMLSchema#

SPARQL Default Graph and Named Graphs

Amazon Neptune associates every triple with a named graph. The default graph is defined as the union of all named graphs.

Default Graph for Queries

If you submit a SPARQL query without explicitly specifying a graph via the GRAPH keyword or constructs such as FROM NAMED, Neptune always considers all triples in your DB instance. For example, the following query returns all triples from a Neptune SPARQL endpoint:

SELECT * WHERE { ?s ?p ?o }

Triples that appear in more than one graph are returned only once.

For information about the default graph specification, see the RDF Dataset section of the SPARQL 1.1 Query Language specification.

Specifying the Named Graph for Loading, Inserts, or Updates

If you don't specify a named graph when loading, inserting, or updating triples, Neptune uses the fallback named graph defined by the URI http://aws.amazon.com/neptune/vocab/v01/DefaultNamedGraph.

When you issue a Neptune Load request using a triple-based format, you can specify the named graph to use for all triples by using the parserConfiguration: namedGraphUri parameter. For information about the Load command syntax, see Neptune Loader Command.

Important

If you don't use this parameter, and you don't specify a named graph, the fallback URI is used: http://aws.amazon.com/neptune/vocab/v01/DefaultNamedGraph.

This fallback named graph is also used if you load triples via SPARQL UPDATE without explicitly providing a named graph target.

You can use the quads-based format N-Quads to specify a named graph for each triple in the database.

Note

Using N-Quads allows you to leave the named graph blank. In this case, http://aws.amazon.com/neptune/vocab/v01/DefaultNamedGraph is used.

You can override the default named graph for N-Quads using the namedGraphUri parser configuration option.

SPARQL XPath Constructor Functions Supported by Neptune

The SPARQL standard allows SPARQL engines to support an extensible set of XPath constructor functions. Neptune currently supports the following constructor functions, where the xsd prefix is defined as http://www.w3.org/2001/XMLSchema#:

  • xsd:boolean

  • xsd:integer

  • xsd:double

  • xsd:float

  • xsd:decimal

  • xsd:long

  • xsd:unsignedLong

xsd:dateTime Values in Neptune

For performance reasons, Neptune always stores date/time values as Coordinated Universal Time (UTC). This makes direct comparisons very efficient.

This also means that if you enter a dateTime value that specifies a particular time zone, Neptune translates the value to UTC and discards that time-zone information. Then, when you retrieve the dateTime value later, it is expressed in UTC, not the time of the original time zone, and you can no longer tell what that original time zone was.

Neptune Handling of Special Floating Point Values

Neptune handles special floating-point values in SPARQL as follows.

SPARQL NaN Handling in Neptune

In Neptune, SPARQL can accept a value of NaN in a query. No distinction is made between signaling and quiet NaN values. Neptune treats all NaN values as quiet.

Semantically, no comparison of a NaN is possible, because nothing is greater than, less than, or equal to a NaN. This means that a value of NaN on one side of a comparison in theory never matches anything on the other side.

However, the XSD specification does treat two xsd:double or xsd:float NaN values as equal. Neptune follows this for the IN filter, for the equal operator in filter expressions, and for exact match semantics (having a NaN in the object position of a triple pattern).

SPARQL Infinite Value Handling in Neptune

In Neptune, SPARQL can accept a value of INF or -INF in a query. INF compares as greater than any other numeric value, and -INF compares as less than any other numeric value.

Two INF values with matching signs compare as equal to each other regardless of their type (for example, a float -INF compares as equal to a double -INF).

Of course, no comparison with a NaN is possible because nothing is greater than, less than, or equal to a NaN.

SPARQL Negative Zero Handling in Neptune

Neptune normalizes a negative zero value to an unsigned zero. You can use negative zero values in a query, but they aren't recorded as such in the database, and they compare as equal to unsigned zeros.

Neptune Limitation of Arbitrary-Length Values

Neptune limits the storage size of XSD integer, floating point, and decimal values in SPARQL to 64 bits. As a result, larger values might be arbitrarily truncated, with unexpected results.

Neptune Extends Equals Comparison in SPARQL

The SPARQL standard defines a ternary logic for value expressions, where a value expression can either evaluate to true, false, or error. The default semantics for term equality as defined in the SPARQL 1.1 specification), which applies to = and != comparisons in FILTER conditions, produces an error when comparing data types that are not explicitly comparable in the operators table in the specification.

This behavior can lead to unintuitive results, as in the following example.

Data:

<http://example.com/Server/1> <http://example.com/ip> "127.0.0.1"^^<http://example.com/datatype/IPAddress>

Query 1:

SELECT * WHERE { <http://example.com/Server/1> <http://example.com/ip> ?o . FILTER(?o = "127.0.0.2"^^<http://example.com/datatype/IPAddress>) }

Query 2:

SELECT * WHERE { <http://example.com/Server/1> <http://example.com/ip> ?o . FILTER(?o != "127.0.0.2"^^<http://example.com/datatype/IPAddress>) }

With the default SPARQL semantics that Neptune used before release 1.0.2.1, both queries would return the empty result. The reason is that ?o = "127.0.0.2"^^<http://example.com/IPAddress> when evaluated for ?o := "127.0.0.1"^^<http://example.com/IPAddress> produces an error rather than false because there are no explicit comparison rules specified for the custom data type <http://example.com/IPAddress>. As a result, the negated version in the second query also produces an error. In both queries, the error causes the candidate solution to be filtered out.

Starting with release 1.0.2.1, Neptune has extended the SPARQL inequality operator in accord with the specification. See the SPARQL 1.1 section on operator extensibility, which allows engines to define additional rules on how to compare across user-defined and non-comparable built-in data types.

Using this option, Neptune now treats a comparison of any two data types that is not explicitly defined in the operator-mapping table as evaluating to true if the literal values and data types are syntactically equal, and false otherwise. An error is not produced in any case.

Using these new semantics, the second query would return "127.0.0.1"^^<http://example.com/IPAddress> instead of an empty result.

Handling of Out-of-Range Literals in Neptune SPARQL

XSD semantics define each numeric type with its value space, except for integer and decimal. These definitions limit each type to a range of values. For example, the range of an xsd:byte range is from -128 to +127, inclusive. Any value outside of this range is considered invalid.

If you try to assign a literal value outside of the value space of a type (for example, if you try to set an xsd:byte to a literal value of 999), Neptune accepts the out-of-range value as-is, without rounding or truncating it. But it doesn't persist it as a numeric value because the given type can't represent it.

That is, Neptune accepts "999"^^xsd:byte even though it is a value outside of the defined xsd:byte value range. However, after the value is persisted in the database, it can only be used in exact match semantics, in an object position of a triple pattern. No range filter can be executed on it because out-of-range literals are not treated as numeric values.

The SPARQL 1.1 specification defines range operators in the form numeric-operator-numeric, string-operator-string, literal-operator-literal, and so forth. Neptune can't execute a range comparison operator anything like invalid-literal-operator-numeric-value.