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Disjunction vs. Optional ... and UNION

From: Bob MacGregor <bmacgregor@siderean.com>
Date: Sun, 20 Mar 2005 09:39:36 -0800
Message-ID: <423DB558.6070709@siderean.com>
To: public-rdf-dawg-comments@w3.org

The committee has shelved disjunction and retained optional.
The reasons for this include the following statement:

> Disjuntion provides a lot of the same capability as optional matches, but
> as a developer I've only seen feature reqests expressed in terms of
> optional match, no disjunction. Many disjunctive queries can be expressed
> in terms of optionals and value disjunctions, but I have not attempted to
> show wether all can be or not.

This statement reveals a PROFOUND ignorance about the meaning and uses of
disjunction and optional.

Backing up a bit first, the right way to design a logic or a language (e.g., SQARQL)
is to invent a small set of core operators, get their meaning right, and then
build up the rest of the language in terms of them.  That way, the semantic
spec for the language is relatively small, even as the syntax grows.  The
base operators for SPARQL ought to be AND and OR (to begin with).

The SQL language provides a few million or billion use cases for disjunction, but
for those who still don't get it, here are a couple of simple examples.

SELECT ?c
WHERE (?c rdf:type my:Car) AND
      { (?c color "red") OR (?c color "blue") }


SELECT ?p
WHERE (?p rdf:type my:Person) AND
      (?p ?age ?a) AND
      { (FILTER ?a < 21) OR (FILTER ?a > 65) }


The correct semantics for OPTIONAL can be illustrated by defining it in terms of
disjunction.  For example:

         WHERE (?a my:foo ?b) (OPTIONAL (?a my:bar ?c))

is equivalent to 

         WHERE (?a my:foo ?b) {(?a my:bar ?c) OR TRUE}

where 'TRUE' is a statement that always evaluates to true.  SPARQL may or may not wish
to include the constant 'TRUE' in its syntax, but 'TRUE' is quite handy for defining
semantics.

So, OPTIONAL is conveniently defined as a disjunction of its argument and the constant TRUE.  
Once you understand this, it is easy to observe that an OPTIONAL clause has no
ability to restrict the bindings in a WHERE clause.

The two use case examples for disjunction that I included above both restrict the
rest of the WHERE clause.  Therefore, they can't possibly be rewritten in terms
of OPTIONAL.


Popping up.  The SPARQL query appears to be deathly afraid of disjunction, while
including OPTIONAL.  The semantics of OPTIONAL, while different, and weaker, than
disjunction, are rather similar with regards to getting a correct, declarative
semantics.  In other words committeed should be as leery of OPTIONAL as they are 
of disjunction  -- I seriously doubt that the committee could get the semantics for OPTIONAL
correct without knowing how to work out a semantics for disjunction.

Therefore, while I have been a vocal supporter of OPTIONAL up to now, I would now
like to reverse my position, and recommend witholding OPTIONAL from the spec until
such time as the committee can successfully revisit the question of disjunction.


That still leaves the unfortunate UNION operator.  SQL has UNION, along with INTERSECTION
and DIFFERENCE (if I recall correctly), and isolates them by placing them outside
of the SELECT-FROM-WHERE block.  Doing so makes a clear syntactic distinction between
UNION and OR.  The SPARQL syntax makes UNION appear to be synonym for the disjunctive
OR operator.  The spec states that UNION works on graphs rather than statements, but its
not at all clear syntactically how the statements in a where clause magically turn into
graphs.  Users will be hopelessly confused by UNION syntax as it now exists.

Therefore, I recommend either eliminating UNION for the present, or redefining it as SQL
has done, placing it outside of the SELECT WHERE block.

Regards, Bob
Received on Sunday, 20 March 2005 17:40:16 GMT

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