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---+ Generating a Unique ID via SPARQL HTTP
Suppose you need unique IDs from within Virtuoso (unique only to a given Virtuoso instance/cluster
would suffice), and that this should be done via SPARQL HTTP prior to issuing a SPARQL insert.
One solution would be to create a table and procedure that keeps track of an incremental ID and
returns each ID only once. Then you can access this procedure via SPARQL HTTP by using:
<verbatim>
(
SELECT ( bif:foo() ) AS ?id
WHERE
{
?s ?p ?o
} limit 1
)
</verbatim>
---++ An optimized solution
Virtuoso's built-in functions (BIFs) <code>[[http://docs.openlinksw.com/virtuoso/fn_sequence_next.html][sequence_next()]]</code>
and <code>[[http://docs.openlinksw.com/virtuoso/sequenceobjects.html][sequence_set()]]</code>
will streamline this task.
Like any other SQL functions without INOUT or OUT parameters, these can be called from SPARQL
simply as <b><code>bif:sequence_next()</code></b> or <b><code>bif:sequence_set()</code></b>,
but an IN parameter may be used to get one sequence per graph, i.e.,
<b><code><nowiki>bif:sequence_next("GRAPH_IDENTIFIER")</nowiki></code></b>, e.g.,
<code><nowiki>bif:sequence_next("<http://my.example.com/graph1>")</nowiki></code>.
<verbatim>
SQL> SPARQL
INSERT INTO GRAPH <http://mygraph.com>
{
<:a> <:p> `bif:sequence_next("<http://mygraph.com>")`
} ;
callret-0
VARCHAR
_______________________________________________________________________________
Insert into <http://mygraph.com>, 1 (or less) triples -- done
1 Rows. -- 141 msec.
SQL> SPARQL
INSERT INTO GRAPH <http://mygraph.com>
{
<:a> <:p> `bif:sequence_next("<http://mygraph.com>")`
} ;
callret-0
VARCHAR
_______________________________________________________________________________
Insert into <http://mygraph.com>, 1 (or less) triples -- done
1 Rows. -- 4 msec.
SQL> SPARQL
SELECT *
FROM <http://mygraph.com>
WHERE
{
?s ?p ?o
}
;
s p o
VARCHAR VARCHAR VARCHAR
_______________________________________________________________________________
:a :p 0
:a :p 1
2 Rows. -- 1 msec.
SQL>
</verbatim>
---++ Side-note -- bif: prefix vs. sql: prefix
For user-defined functions, the <b><code>sql:</code></b> prefix is preferred over
the <b><code>bif:</code></b> prefix; e.g.,
<verbatim>
sql:foo(x)
</verbatim>
-- will call --
<verbatim>
DB.DBA.foo(x)
</verbatim>
The difference is that <code>bif:foo</code> will make an unqualified call to <code>foo()</code>.
If no built-in function <code>foo()</code> exists, and the system contains many users and database
qualifiers, the search for exact name of <code>XXX.YYY.foo()</code> adds needless overhead.
Further issues may arise if multiple <code>XXX.YYY.foo()</code> procedures have been defined.
---++ Related
* [[VirtSPARQLOptGuide][Virtuoso SPARQL Optimization Guide]]
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