Re: [PRD/FLD] aggregates

On Sun, 12 Oct 2008 17:25:50 -0700
Gary Hallmark <gary.hallmark@oracle.com> wrote:

> 
> Hi Michael,
> 
> Thanks for getting the ball rolling.  I think this should work for PRD, 
> but let me check that I understand your proposal.  An example rule for 
> computing the average salary of employees grouped by department would be 
> something like:
> 
> Forall ?deptno ?sal ?empId (
>   AvgDeptSal(?deptno avg(?sal [ ?deptno ] | Emp(?empId ?deptno ?sal)))
> )

I am not sure how aggregates are supposed to be used in PRD, but:

  - in a logical rule-based language they would be in the body of a rule.
  - the comprehension variable is not quantified
    (the aggregate works as a kind of quantifier for it)

So, in such a language I would write something like

Forall ?depno ?Avgsal (
  Query(?depno ?Avgsal) :-
	?Avgsal = avg(?sal [?deptno] | Exists ?empId (Emp(?empId ?deptno ?sal)))
)

I suppose this can also be written as a fact like yours:

Forall ?deptno (
   AvgDeptSal(?deptno avg(?sal [?deptno] | Exists ?empId Emp(?empId ?deptno ?sal)))
)

but I haven't thought about it.



> And if PRD doesn't support group by (I don't know of any PR engines that 
> do), we can simulate using
> 
> Deptno(?deptno) :- Emp(?empId ?deptno ?sal)
> AvgDeptSal(?deptno ?avgSal) :- And( Deptno(?deptno) ?avgSal = avg(?sal | 
> Emp(?empId ?deptno ?sal)))

Something like that. But you do not need to simulate anything. You just do not
include the groupby variables in PRD. The syntax that I proposed is for FLD and
the dialects that will extend BLD in the future. This is not even in BLD (or
core).


> Are the aggfuns the usual min, max, sum, avg, count?  (BTW, I don't 
> think count needs a Var). 

Yes. (For FLD we should allow whatever a future dialect might want to have.)
Regarding count, you do not need a var but for uniformity you can use a ?.

> Also nice to include list as an aggfun, that just returns a list of var 
> bindings.  (Of course, we need to add lists)

Strictly speaking this is unnecessary since we can write

      agg{?V ...| And(query ?V=some-list)}


michael


> Michael Kifer wrote:
> > I sent this message at the end of the last f2f, but it doesn't seem to have been
> > delivered. Only today I got a reply that it was rejected by the server.
> > Anyway, here it goes again:
> >
> >     Since we did not have time to discuss the aggregates, let's start it by
> >     email.  Basically, an aggregate is a term that includes a comprehension.
> >     In addition, there is a need to be able to GROUP BY, as in databases.  (I
> >     do not know if this latter thing is needed for PRD.)
> >
> >     So, the syntax I was thinking about is:
> >
> > 	aggfun{Var [ GroupvarList ] | CondFormula}
> >
> >     The symbols {,},[,],| here are the actual symbols, not metasymbols.
> >     Var is the comprehension variable, i.e., {Var | CondFormula}.
> >     GroupvarList is the list of vars to group by. The entire piece
> >     "[ GroupvarList ]" is optional.
> >
> >     Note that I need the above general form for FLD. For PRD we might need
> >     something less general. We just need to make sure that the
> >     syntaxes are compatible.
> >
> >
> > 	    --michael  
> >
> >   
> 
>

Received on Monday, 13 October 2008 04:08:23 UTC