Checkpoint-Bounded Scans

When background machinery repeatedly scans a table by a monotonic column — 'everything with timestamp ≤ now' — maintain a persistent checkpoint of the last position processed and bound every scan from below as well as above. The unbounded form forces the storage engine to consider every pending row on every pass (in MVCC systems like MySQL, walking a history list that grows with backlog); the bounded form touches only the increment since the last pass, keeping scan cost proportional to progress rather than to backlog depth.

The pattern is what makes a buffer's performance independent of how full it is — the property any queue, scheduler, or expiry sweeper needs if its whole purpose is absorbing backlogs. The same shape appears as watermarks in stream processors and incremental cursors in reconciliation jobs.