Aggregation

FlowLog supports aggregate functions in rule heads. Aggregations compute summary values over groups of tuples.

Syntax

An aggregate function wraps an arithmetic expression in the head of a rule:

Head(group_key, agg_op(value_expr)) :- Body(...).

Columns in the head that are not wrapped in an aggregate automatically become group-by keys.

Supported operators

Operator	Description
min / MIN	Minimum value
max / MAX	Maximum value
sum / SUM	Sum of values
count / COUNT	Count of tuples
average / AVG	Average value

Examples

Connected components

Compute the connected component label for each node as the minimum reachable node id:

.decl Arc(a: int32, b: int32)
.input Arc(IO="file", filename="Arc.csv", delimiter=",")

.decl CC(node: int32, cc: int32)
.printsize CC

CC(node, min(node)) :- Arc(node, _).
CC(node, min(cc)) :- Arc(other, node), CC(other, cc).

Here node is the group-by key and min(...) selects the smallest component label.

Single-source shortest paths

Compute shortest distances from seed nodes using min aggregation:

.decl arc(src: int32, dest: int32, weight: int32)
.decl id(src: int32)
.decl sssp(x: int32, y: int32)

.input arc(IO="file", filename="Arc.csv", delimiter=",")
.input id(IO="file", filename="Id.csv", delimiter=",")

sssp(x, min(0)) :- id(x).
sssp(y, min(d1 + d2)) :- sssp(x, d1), arc(x, y, d2).

.printsize sssp

The aggregate argument can be an arithmetic expression (d1 + d2).

Aggregation and stratification

Aggregated relations participate in FlowLog's stratification analysis. The compiler schedules evaluation strata so that aggregated values are fully computed before downstream rules consume them.