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Aggregations
Aggregations in Cozo can be thought of as a function that acts on a string of values and produces a single value (the aggregate). Due to Datalog semantics, the stream is never empty.
There are two kinds of aggregations in Cozo, meet aggregations and normal aggregations. Meet aggregations satisfy the additional properties of idempotency: the aggregate of a single value a is a itself, commutivity: the aggregate of a then b is equal to the aggregate of b then a, and commutivity: it is immaterial where we put the parentheses in an aggregate application. They are implemented differently in Cozo, with meet aggregations faster and more powerful (only meet aggregations can be recursive).
Meet aggregations can be used as normal ones, but the reverse is impossible.
Meet aggregations
min(x): aggregate the minimum value of all x.
max(x): aggregate the maximum value of all x.
and(var): aggregate the logical conjunction of the variable passed in.
or(var): aggregate the logical disjunction of the variable passed in.
union(var): aggregate the unions of var, which must be a list.
intersection(var): aggregate the intersections of var, which must be a list.
choice(var), choice_last(var): non-deterministically chooses one of the values of var as the aggregate. The first one simply chooses the first value it meets (the order that it meets values should be considered non-deterministic), whereas the second one chooses the last value.
min_cost(var): var should be a list of two elements: [data, cost], and this aggregation chooses the list of the minimum cost.
shortest(var): var must be a list. Returns the shortest list among all values. Ties will be broken non-deterministically.
coalesce(var): returns the first non-null value it meets. The order is non-deterministic.
bit_and(var): var must be bytes. Returns the bitwise 'and' of the values.
bit_or(var): var must be bytes. Returns the bitwise 'or' of the values.
Normal aggregations
count(var): count how many values are generated for var (using bag instead of set semantics).
count_unique(var): count how many unique values there are for var.
collect(var): collect all values for var into a list.
unique(var): collect var into a list, keeping each unique value only once.
group_count(var): count the occurrence of unique values of var, putting the result into a list of lists, e.g. when applied to 'a', 'b', 'c', 'c', 'a', 'c', the results is [['a', 2], ['b', 1], ['c', 3]].
bit_xor(var): var must be bytes. Returns the bitwise 'xor' of the values.
Statistical aggregations
mean(x): the mean value of x.
sum(x): the sum of x.
product(x): the product of x.
variance(x): the sample variance of x.
std_dev(x): the sample standard deviation of x.