Frame

Aka limit/offset

Signature

frame(operand: Relation, order: Ordering, offset: Integer, limit: Integer) -> Relation

Examples

frame(suppliers, [:status, :sid], 0, 3)

frame(suppliers, [[:status, :asc], [:sid, :desc]], 1, 2)

Description

Computes a relation by restricting the tuples of operand to a particular frame. This frame can be easily remembered through the "skip offset, take 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 deterministic, 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.

Implementation notes

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 order.