DeAL TutorialUser-Defined Aggregates

User-Defined Aggregates (.deal | .fac)

The user can define a new aggregate, called newaggr, by writing the rules that support:

single(newaggr, Y, NV)

and

multi(newaggr, Y, OV, NV)

where

  • single is the computation to be performed on a singleton set having Y as its only element.
  • multi denotes how to compute the aggregate value NV for a set S' obtained by adding a new element Y to the the set S, where the value of the aggregate for S is OV.

For instance, if we had to define the max aggregate we would write:

single(max, Y, Y).
multi(max, Y, MO, MN) <- Y > MO, MN = Y.
multi(max, Y, MO, MN) <- Y <= MO, MN = MO.

Likewise, in DeAL, count and sum could have been defined as shown by the two pairs of rules below:

single(count,  Y, 1).
multi(count, Y, Old, New) <- New = Old + 1.

single(sum, Y, Y).
multi(sum, Y, Old, New) <- New = Old + Y.

The count and sum so defined behave as count_all and sum_all since these rules accumulate the Old value with the new Y, without checking whether the same Y value had already occurred.

User-defined aggregates can also be called by means of aggr goals. In this case, when applied to the empty set, the compiler will search for an empty rule defining the behavior of that particular aggregate on an empty set. For instance, the built-in aggregates behave as if they were defined by the following rules:

empty(sum, 0).
empty(count, 0).
empty(max, 0) <- false.

On empty set, aggr will return 0 for sum and count, and will fail on max (also fails on min and avg).

Several new aggregates can be defined using the single and multi rules. For instance in SQL, after the maximum is found, a second sub-query is needed to return all the values associated with the maximum. In DeAL, if sppp denotes a supplier-part-price relation, to find, for each supplier their most expensive items and their common price of these items, we can write:

findmax(S, mymax<(Itm, Pric)>) <- sppp(S, Itm, Pric).

single(mymax, (Item, Pr), (Item, Pr)).

multi(mymax, (Sit, Sp), (Oit, Op), (Sit, Sp)) <- Sp >= Op.
multi(mymax, (Sit, Sp), (Oit, Op), (Oit, Op)) <- Sp < Op.

This example illustrates that:

  1. Aggregates can return full tuples, such as the pair (Item, Pr) produced by mymax
  2. More than one value/tuple can be returned from the computation of an aggregate.

Last Updated 2014