Exercise 2: An Accidentally Unsorted EAST-TXN-FILE Going Into MERGE — Possible Solution ==================================================================== WHAT MERGE GENUINELY ASSUMES ABOUT ITS OWN INPUT FILES This chapter's own warn-box states the real precondition directly: MERGE combines files that are already sorted on the same real key - it never sorts anything itself. The entire real algorithm MERGE relies on depends on both EAST-TXN-FILE and WEST-TXN-FILE genuinely already being in ascending TXN-CUSTOMER-ID order before the MERGE statement even begins. WHAT GENUINELY HAPPENS IF EAST-TXN-FILE IS ACCIDENTALLY UNSORTED Since MERGE's own real algorithm is built entirely around combining two already-ordered streams by comparing their next available records against each other, feeding it a genuinely unsorted EAST-TXN-FILE breaks that core assumption. The real output, COMBINED-TXN-FILE, comes out in a genuinely wrong order - MERGE proceeds exactly as if both inputs were correctly sorted, since it has no real way of knowing otherwise, and the resulting sequence simply doesn't reflect a proper merge of the two real sources. WHY MERGE ITSELF GENUINELY WOULDN'T CATCH THIS MISTAKE This chapter's own warn-box states this plainly: MERGE "doesn't detect the problem or fix it." There's no real verification step built into MERGE that checks whether its own input files are actually sorted before combining them - it simply trusts that precondition and executes its real combining algorithm regardless of whether that trust is genuinely warranted. The statement completes without error, but the real result is silently incorrect. WHY THIS IS A GENUINELY DANGEROUS FAILURE MODE Because MERGE runs to completion with no error message at all, this kind of mistake can go completely unnoticed unless someone separately verifies COMBINED-TXN-FILE's own real ordering afterward. A silent, successful-looking statement producing a genuinely wrong result is a more dangerous real failure mode than one that visibly crashes, since nothing about the program's own behavior signals that anything went wrong. ANSWER: If EAST-TXN-FILE were accidentally left unsorted before this MERGE statement ran, the real output, COMBINED-TXN-FILE, would come out in a genuinely incorrect order, since MERGE's whole real algorithm depends on both input files already being sorted by TXN-CUSTOMER-ID - MERGE itself never sorts anything, it only combines streams that are already in order. MERGE wouldn't catch this mistake because it has no real verification step checking whether its inputs are actually sorted; it simply trusts that precondition and executes normally, producing a silently wrong result with no error raised at all. WHY THIS WORKS AS AN ANSWER ------------------------------ This correctly identifies the specific real consequence (wrong output order, not a crash or error) and explains why MERGE has no mechanism to detect the violated precondition, grounding both points directly in this chapter's own warn-box.