9. Managing Tuple Tables

As explained in Section 4, a data store uses tuple tables as containers for facts – that is, triples and other kind of data that RDFox should process. Each tuple table is identified by a name that is unique for a data store. Moreover, each tuple table has a minimal and maximal arity, which are numbers determining the smallest and the largest numbers of RDF resources in a fact stored in the tuple table. In most cases, the minimal and maximal arity are the same, in which case they are called just arity.

9.1. Types of Tuple Tables

RDFox supports three kinds of tuple tables.

  • In-memory tuple tables are most commonly used kind of tuple table, which, as the name suggests, store facts in RAM. RDFox uses in-memory tuple tables of arity three to store triples of the default graph and the named graphs of RDF. In particular, an in-memory tuple table called http://oxfordsemantic.tech/RDFox#DefaultTriples is created automatically when a fresh data store is created, and RDFox will create additional in-memory tuple tables for each named graph it encounters. RDFox provides ways to add and delete facts in in-memory tuple tables.

  • Data source tuple tables provide a ‘virtual view’ over data in non-RDF data sources, such as CSV files, relational databases, or a full-text Solr index. Such tuple tables must be created explicitly by the user, and doing so requires specifying how the external data is to be transformed into a format compatible with RDF. The facts in data source tuple tables are ‘virtual’ in the sense that they are constructed automatically by RDFox based on the data in the data source — that is, there is no way to add/delete such facts directly. Finally, data source tuple tables can be of arbitrary arity — that is, such tuple tables are not limited to containing just triples. Data source tuple tables and the process of importing external data are described in detail in Section 10.

  • Built-in tuple tables contain some well-known facts that can be useful in various applications of RDFox. The facts in such tuple tables cannot be modified by users; rather, they are produced on the fly by RDFox as needed. They are described in more detail in Section 9.5.

9.2. Fact Domains

Each fact in a tuple is associated with one or more fact domains.

  • The EDB fact domain contains facts that were imported explicitly by the user. The name EDB is an abbreviation of Extensional Database.

  • The IDB fact domain contains facts that were derived using rules. The name IDB is an abbreviation of Intensional Database. This fact domain is used as the default in all operations that take a fact domain as argument.

  • The IDBrep fact domain contains the representative facts of the IDB domain. This fact domain differs only in data stores for which equality reasoning (i.e., reasoning with owl:sameAs) is turned on.

  • The IDBrepNoEDB fact domain contains facts of the IDB domain that are not in the EDB domain, which are essentially facts that were derived during reasoning and were not present in the input.

A fact can belong to more than one domain. For example, facts added to the store are stored into the EDB domain, and during reasoning they are transferred into the IDB domain.

Only the EDB fact domain can be directly affected by users. That is, all explicitly added facts are added to the EDB domain, and only those facts can be deleted. It is not possible to manually delete derived facts since the meaning of such deletions is unclear.

Many RDFox operations accept a fact domain as an argument. For example, SPARQL query evaluation takes a fact domain as an argument, which determines what subset of the facts the query should be evaluated over. Thus, if a query is evaluated with respect to the EDB domain, it will ‘see’ only the facts that were explicitly added to a data store, and it will ignore the the facts that were derived by reasoning.

9.3. Managing and Using Tuple Tables

RDFox provide ways for creating and deleting tuple tables: this can be accomplished in the shell using the tupletable command (see Section 15.2.2.40), and the relevant APIs are described in Section 13.6. When creating a tuple table, one must specify a list of key-value parameters that determine what kind of tuple table is to be created. The parameters for data source tuple tables depend on the type of data source and are described in detail in Section 10. Moreover, the parameters for in-memory and built-in tuple tables are described in Section 9.4 and Section 9.5, respectively.

RDFox provides ways to add and delete facts to in-memory tuple tables: this can be accomplished in the shell using the import command (see Section 15.2.2.20), and the relevant APIs are described in Section 13.4.5.

Facts in a tuple table can be accessed during querying and reasoning. In queries, tuple tables corresponding to the default graph and the named graphs can be accessed using standard SPARQL syntax for triple patterns and the GRAPH operator — that is, a triple pattern outside a GRAPH operator will access the http://oxfordsemantic.tech/RDFox#DefaultTriples tuple table, and a triple pattern inside a GRAPH :G operator will access the in-memory tuple table with name :G. To access tuple tables of other types, RDFox extends the SPARQL syntax with the TT operator, which is described in Section 5.3. Note that the default graph and the named graphs can also be accessed using the TT operator. Moreover, tuple tables can be accessed in rules using the general atom syntax described in Section 6.4.1.3. Since only in-memory tuple tables can be modified by users, any atom occurring in the head of a rule is allowed to mention only an in-memory tuple table.

9.4. In-Memory Tuple Tables

RDFox uses in-memory tuple tables to store facts imported by the users. At present, RDFox supports only tuple tables of arity three, thus allowing the system to store only triples. An in-memory tuple table called http://oxfordsemantic.tech/RDFox#DefaultTriples is created automatically when a fresh data store is created. Moreover, in-memory tuple tables can be created in the following three ways.

  • When instructed to import data containing triples in graphs other than the default one, RDFox will automatically create a tuple table for each named graph it encounters.

  • The SPARQL 1.1 Update command CREATE GRAPH creates an in-memory tuple table for each named graph.

  • In-memory tuple tables can be created using tuple table management APIs. The main benefit of this over the above two methods is the ability to specify additional parameters, as described in the following table.

Parameter

Default value

Description

type

triples

Specifies that the tuple table will be used to store triples — that is, the tuple table backs the default or a named graph. This parameter must be specified when creating an in-memory tuple table.

max-triple-capacity

(as in the data store)

Specifies the maximum number of triples that the new tuple table will be able to hold. The main purpose of this parameter is to reduce the amount of address space that the tuple table will use. The default value for is the value of the data store parameter with the same name.

init-triple-capacity

(as in the data store)

Provides a hint as to how many facts the system should expect to store initially in the tuple table. When importing large data sets, setting this parameter to be roughly equal to the number of facts to be imported can significantly improve the speed of importation.

9.5. Built-In Tuple Tables

Built-in tuple tables are similar to built-in functions; however, whereas a built-in function returns just one value for a given number of arguments, a built-in tuple table can relate sets of values. Thus, facts in built-in tuple tables are not stored explicitly; rather, they are produced on the fly as query and/or rule evaluation progresses. Other than this internal detail, built-in tuple tables are used in queries and rules just like any other tuple table: they are referenced in queries using the proprietary TT operator (see Section 5.3), and they are referenced in rules using general atoms (see Section 6.4.1.3). Built-in tuple tables are the only ones for which the minimal and the maximal arity are not necessarily the same.

Each built-in tuple table is identified by a well-known name, which cannot be changed. The names of all of built-in tuple tables starts with http://oxfordsemantic.tech/RDFox#, which is abbreviated in the rest of this section as rdfox:. For example, the rdfox:SKOLEM built-in tuple table is always available under that name. When a data store is created, all built-in tuple tables supported by RDFox will be created automatically. It is very unlikely that users will ever need to delete built-in tuple tables; nevertheless, for the sake of consistency, RDFox allows such tuple tables to be deleted just like any other tuple table. In case a built-in tuple table is deleted, it can be recreated using standard methods, by simply specifying the tuple table name without any parameters. (Please note that, as a consequence of this, it is not possible to create an in-memory or a data source tuple table with a name that is reserved for a built-in tuple table.)

9.5.1. rdfox:SKOLEM

The rdfox:SKOLEM tuple table can have arity from one onwards. Moreover, in each fact in this tuple table, the last resource of the fact is a blank node that is uniquely determined by all remaining arguments. This can be useful in queries and/or rules that need to create new objects. This is explained using the following example.

Example: Let us assume we are dealing with a dataset where each person is associated with zero or more companies using the :worksFor relationship. For example, our dataset could contain the following triples.

:Peter :worksFor :Company1 .
:Peter :worksFor :Company2 .
:Paul  :worksFor :Company1 .

Now assume that we wish to attach additional information to each individual employment. For example, we might want to say that the employment of :Peter in :Company1 started on a specific date. To be able to capture such data, we will ‘convert’ each :worksFor link to a separate instance of the :Employment class; then, we can attach arbitrary information to such instances. This presents us with a key challenge: for each combination of a person and company, we need to ‘invent’ a fresh object that is uniquely determined by the person and company.

This problem is solved using the rdfox:SKOLEM built-in tuple table. In particular, we can restructure the data using the following rule.

:Employment[?E], :employee[?E,?P], :inCompany[?E,?C] :- :worksFor[?P,?C], rdfox:SKOLEM("Employment",?P,?C,?E) .

The above rule can be understood as follows. Body atom :worksFor[?P,?C] selects all combinations of a person and a company that the person works for. Moreover, atom rdfox:SKOLEM("Employment",?P,?C,?E) contains all facts where the value of ?E is uniquely determined by the fixed string "Employment", the value of ?P, and the value of ?C. Thus, for each combination of ?P and ?C, the built-in tuple table will produce a unique value of ?E, which is then used in the rule head to derive new triples.

How a value of ?E is computed from the other arguments is not under application control: each value is a blank node whose name is guaranteed to be unique. However, what matters is that the value of ?E is always the same whenever the values of all other arguments are the same. Thus, we can use the following rule to specify the start time of Peter’s employment in Company 1.

:startDate[?E,"2020-02-03"^^xsd:date] :- rdfox:SKOLEM("Employment",:Peter,:Company1,?E) .

After evaluating these rules, the following triples will be added to the data store. We use blank node names such as _:new_1 for clarity: the actual names of new blank nodes will me much longer in practice.

_:new_1 rdf:type   :Employment            .
_:new_1 :employee  :Peter                 .
_:new_1 :inCompany :Company1              .
_:new_1 :startDate "2020-02-03"^^xsd:date .
_:new_2 rdf:type   :Employment            .
_:new_2 :employee  :Peter                 .
_:new_2 :inCompany :Company2              .
_:new_3 rdf:type   :Employment            .
_:new_3 :employee  :Paul                  .
_:new_3 :inCompany :Company1              .

When creating fresh objects using the rdfox:SKOLEM built-in tuple table, it is good practice to incorporate object type into the argument. The above example achieved this by passing a fixed string "Employment" as the first argument of rdfox:SKOLEM. This allows us to create another, distinct blank node for each combination of a person and a company by simply varying the first argument of rdfox:SKOLEM.

Atoms involving the rdfox:SKOLEM built-in tuple table must satisfy certain binding restrictions in rules and queries. Essentially, it must be possible to evaluate a query/rule so that, once an rdfox:SKOLEM atom is reached, either the value of the last argument, or the values of all all but the last argument must be known. This is explained using the following example.

Example: The following query cannot be evaluated by RDFox — that is, the system will respond with a query planning error.

SELECT ?P ?C ?E WHERE { TT rdfox:SKOLEM { "Employment" ?P ?C ?E } }

This query essentially says “return all ?P, ?C, and ?E where the value of ?E is uniquely defined by "Employment", ?P, and ?C”. The problem with this is that the values of ?P and ?C have not been restricted in any way, so the query should, in principle, return infinitely many answers.

To evaluate the query, one must provide the values of ?P and ?C, or for ?E, either explicitly as arguments or implicitly by binding the arguments in other parts of the query. Thus, both of the following queries can be successfully evaluated.

SELECT ?E WHERE { TT rdfox:SKOLEM { "Employment" :Paul :Company2 ?E } }
SELECT ?T ?C ?P WHERE { TT rdfox:SKOLEM { ?T ?C ?P _:new_1 } }

The latter query aims to unpack _:new_1 into the values of ?T, ?C, and ?P for which _:new_1 is the uniquely generated fresh blank node. Note that such ?T, ?C, and ?P may or may not exist, depending on the algorithm RDFox uses to generate blank nodes. The following is a more realistic example of blank node ‘unpacking’.

SELECT ?T ?C ?P WHERE { ?E rdf:type :Employment . TT rdfox:SKOLEM { ?T ?C ?P ?E } }