Hi All,
As usual, our friend Osvaldo is making me think in new
directions. We've had some conversations lately about how Rete and
RDBMS technology and concepts overlap. It seems to me that one very
interesting way to answer this question is to make use of this
parallelism and make it more explicit. In response to this question,
I've been thinking about a new construct called "defquery", which
looks something like this:
(defquery my-query (Attribute (name ?aName)))
A defquery is very much like a JDBC "PreparedStatement," and also like
a Jess defrule (that has only one pattern on the LHS, and no RHS.) You
use them by calling execute-query:
(execute-query my-query ?aName Bob)
The result of this might be a multifield
(1 3 46 345 6456)
containing fact-ids of facts that matched the query; or maybe it
contains Fact objects as external-address objects; or perhaps there's
no multifield at all, only a Java Enumeration. In any case, then the
question below can be answered by defining the appropriate query,
executing the query, counting the matches (therte might even be a
function which does the couting for you.)
The implementation of this is very simple. Basically a defquery is a
variation of a defrule that doesn't get put on the agenda. If you're
familiar with Jess' internals, instead of a NodeTerm, a defquery would
have a NodeQuery or some such, but otherwise would be quite the
same. The query execution would be handled by adding some patterns to
the LHS and then asserting some magic query facts; this is similar to
the kinds of tricks used to do backwards chaining.
Anyway, this lets you use the Rete network and its indexing
capabilities to do things a relational database is normally used
for. I can think of plenty of ways to use this.
I certainly won't include anything like this in the 5.0a6 release,
which I want to send out this week. But I'll see about rigging it up
for the one after that.
I think Osvaldo Pinali Doederlein wrote:
>
> Hi,
>
> I'm working with metrics, so I want to know the _number_ of facts that match
> some given pattern. Example:
>
> (Attribute (name ?aName))
>
> If there are 10 Attribute facts with the same value for the slot 'name',
> e.g. "address", I want to assert a new fact
>
> (nameCount (name "address") (count 10))
>
> Is this possible? :) Sorry if I missed something obvious in the docs... I
> guess I can do a very dumb implementation where I match the pattern and
> increment a global variable, but I'm going to do lots of metrics so I hope
> there's a cleaner and more efficient way to do this.
>
> A+
> Osvaldo
>
> ----------------------------------------------------------------
> *** The Free, Open and Pure Java Shell *** Current: Alpha7
> http://www.geocities.com/ResearchTriangle/Node/2005/java_shell.htm
> http://student.vub.ac.be/~opinalid/java_shell.htm
> ----------------------------------------------------------------
> Osvaldo Pinali Doederlein [EMAIL PROTECTED]
> MSc student, Developer, Java & CORBA evangelist, Rainmaker, etc.
>
>
>
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---------------------------------------------------------
Ernest Friedman-Hill
Distributed Systems Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
Org. 8920, MS 9214 [EMAIL PROTECTED]
PO Box 969 http://herzberg.ca.sandia.gov
Livermore, CA 94550
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