frame(operand: Relation, order: Ordering, offset: Integer, limit: Integer) -> Relation
frame(suppliers, [:status, :sid], 0, 3)
frame(suppliers, [[:status, :asc], [:sid, :desc]], 1, 2)
Computes a relation by restricting the tuples of
operand to a particular
frame. This frame can be easily remembered through the "skip
limit" mnemonic mean, provided
order is a total order.
Formally, the frame is defined by those tuples whose ranking according to
order is such that
offset <= rank < limit. In other words, this
operator is actually equivalent to the following definition:
def frame(operand, order, offset, limit) allbut( restrict( rank(operand, order, :rank), lte(offset, :rank) & lt(:rank, offset+limit)), [:rank]) end frame(suppliers, [:city, :sid], 2, 3)
As of current Alf version, for this operator to be semantically sound and
order MUST be a total order, that is, it must at least
cover a candidate key. As of current Alf version, no error is raised if
this is not the case but that might change in future versions.
Contrary to the longer expression shown above, this operator compiles to 'efficient' SQL (rank does not, so far) at the cost of having to provide a total order.
As the result is a relation and relations are not ordered by definition,
the order in which tuples can be observed in the result (e.g. through
explicit tuple iteration, casting to an array, json encoding) is NOT
guaranteed to follow