FAILURE MAP
← Case archive

FA-44971 / Data systems / Open access

Merge join matches only the first row of a repeated left key · case 01

Merge join matches only the first row of a repeated left key.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

merge-join-runs: Merge join matches only the first row of a repeated left key.

THE FAILURE

merge-join-runs: Merge join matches only the first row of a repeated left key.

Unsuccessful approach: Comparing whole rows rather than keys breaks runs with distinct row IDs.

Case contract

Join two key-sorted non-null relations [key,id] by matching equal-key runs. Emit the full left-major Cartesian product for each matching run. Advance the lower unmatched run and preserve physical order within equal-key runs.

Why this case matters

A bounded deterministic data engine model makes representation and changelog faults reproducible.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(d):
    try:
        left,right=d
        i=j=0; out=[]
        while i<len(left) and j<len(right):
            a=left[i][0]; b=right[j][0]
            if a<b: i+=1; continue
            if a>b: j+=1; continue
            ie=i+1
            while False: ie+=1
            je=j+1
            while je<len(right) and right[je][0]==b: je+=1
            for x in left[i:ie]:
                for y in right[j:je]: out.append([x[1],y[1]])
            i,j=ie,je
        return out
    except (IndexError, KeyError, ValueError, StopIteration) as exc:
        return {"representation_error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
if N == 1:
    check('duplicate runs', solve([[[1, 10], [1, 11]], [[1, 20], [1, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[1, 10], [2, 11]], [[2, 20]]]), [[11, 20]])
    check('right gap', solve([[[2, 10]], [[1, 20], [2, 21]]]), [[10, 21]])
    check('single match', solve([[[1, 10]], [[1, 20]]]), [[10, 20]])
    check('no overlap', solve([[[1, 10]], [[3, 20]]]), [])
    check('empty left', solve([[], [[1, 20]]]), [])
    check('empty right', solve([[[1, 10]], []]), [])
elif N == 2:
    check('duplicate runs', solve([[[2, 10], [2, 11]], [[2, 20], [2, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[2, 10], [3, 11]], [[3, 20]]]), [[11, 20]])
    check('right gap', solve([[[3, 10]], [[2, 20], [3, 21]]]), [[10, 21]])
    check('single match', solve([[[2, 10]], [[2, 20]]]), [[10, 20]])
    check('no overlap', solve([[[2, 10]], [[4, 20]]]), [])
    check('empty left', solve([[], [[2, 20]]]), [])
    check('empty right', solve([[[2, 10]], []]), [])
elif N == 3:
    check('duplicate runs', solve([[[3, 10], [3, 11]], [[3, 20], [3, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[3, 10], [4, 11]], [[4, 20]]]), [[11, 20]])
    check('right gap', solve([[[4, 10]], [[3, 20], [4, 21]]]), [[10, 21]])
    check('single match', solve([[[3, 10]], [[3, 20]]]), [[10, 20]])
    check('no overlap', solve([[[3, 10]], [[5, 20]]]), [])
    check('empty left', solve([[], [[3, 20]]]), [])
    check('empty right', solve([[[3, 10]], []]), [])
elif N == 4:
    check('duplicate runs', solve([[[4, 10], [4, 11]], [[4, 20], [4, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[4, 10], [5, 11]], [[5, 20]]]), [[11, 20]])
    check('right gap', solve([[[5, 10]], [[4, 20], [5, 21]]]), [[10, 21]])
    check('single match', solve([[[4, 10]], [[4, 20]]]), [[10, 20]])
    check('no overlap', solve([[[4, 10]], [[6, 20]]]), [])
    check('empty left', solve([[], [[4, 20]]]), [])
    check('empty right', solve([[[4, 10]], []]), [])
elif N == 5:
    check('duplicate runs', solve([[[5, 10], [5, 11]], [[5, 20], [5, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[5, 10], [6, 11]], [[6, 20]]]), [[11, 20]])
    check('right gap', solve([[[6, 10]], [[5, 20], [6, 21]]]), [[10, 21]])
    check('single match', solve([[[5, 10]], [[5, 20]]]), [[10, 20]])
    check('no overlap', solve([[[5, 10]], [[7, 20]]]), [])
    check('empty left', solve([[], [[5, 20]]]), [])
    check('empty right', solve([[[5, 10]], []]), [])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
duplicate runs[[10, 20], [10, 21]][[10, 20], [10, 21], [11, 20], [11, 21]]Failed
left gap[[11, 20]][[11, 20]]Passed
right gap[[10, 21]][[10, 21]]Passed
single match[[10, 20]][[10, 20]]Passed
no overlap[][]Passed
empty left[][]Passed
empty right[][]Passed

SHA-256 / e7d52b8960ddc5287a8a502467052d16d33a897445d7bf76bf4414f7682b72bf

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(d):
    try:
        left,right=d
        i=j=0; out=[]
        while i<len(left) and j<len(right):
            a=left[i][0]; b=right[j][0]
            if a<b: i+=1; continue
            if a>b: j+=1; continue
            ie=i+1
            while ie<len(left) and left[ie]==left[i]: ie+=1
            je=j+1
            while je<len(right) and right[je][0]==b: je+=1
            for x in left[i:ie]:
                for y in right[j:je]: out.append([x[1],y[1]])
            i,j=ie,je
        return out
    except (IndexError, KeyError, ValueError, StopIteration) as exc:
        return {"representation_error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
if N == 1:
    check('duplicate runs', solve([[[1, 10], [1, 11]], [[1, 20], [1, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[1, 10], [2, 11]], [[2, 20]]]), [[11, 20]])
    check('right gap', solve([[[2, 10]], [[1, 20], [2, 21]]]), [[10, 21]])
    check('single match', solve([[[1, 10]], [[1, 20]]]), [[10, 20]])
    check('no overlap', solve([[[1, 10]], [[3, 20]]]), [])
    check('empty left', solve([[], [[1, 20]]]), [])
    check('empty right', solve([[[1, 10]], []]), [])
elif N == 2:
    check('duplicate runs', solve([[[2, 10], [2, 11]], [[2, 20], [2, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[2, 10], [3, 11]], [[3, 20]]]), [[11, 20]])
    check('right gap', solve([[[3, 10]], [[2, 20], [3, 21]]]), [[10, 21]])
    check('single match', solve([[[2, 10]], [[2, 20]]]), [[10, 20]])
    check('no overlap', solve([[[2, 10]], [[4, 20]]]), [])
    check('empty left', solve([[], [[2, 20]]]), [])
    check('empty right', solve([[[2, 10]], []]), [])
elif N == 3:
    check('duplicate runs', solve([[[3, 10], [3, 11]], [[3, 20], [3, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[3, 10], [4, 11]], [[4, 20]]]), [[11, 20]])
    check('right gap', solve([[[4, 10]], [[3, 20], [4, 21]]]), [[10, 21]])
    check('single match', solve([[[3, 10]], [[3, 20]]]), [[10, 20]])
    check('no overlap', solve([[[3, 10]], [[5, 20]]]), [])
    check('empty left', solve([[], [[3, 20]]]), [])
    check('empty right', solve([[[3, 10]], []]), [])
elif N == 4:
    check('duplicate runs', solve([[[4, 10], [4, 11]], [[4, 20], [4, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[4, 10], [5, 11]], [[5, 20]]]), [[11, 20]])
    check('right gap', solve([[[5, 10]], [[4, 20], [5, 21]]]), [[10, 21]])
    check('single match', solve([[[4, 10]], [[4, 20]]]), [[10, 20]])
    check('no overlap', solve([[[4, 10]], [[6, 20]]]), [])
    check('empty left', solve([[], [[4, 20]]]), [])
    check('empty right', solve([[[4, 10]], []]), [])
elif N == 5:
    check('duplicate runs', solve([[[5, 10], [5, 11]], [[5, 20], [5, 21]]]), [[10, 20], [10, 21], [11, 20], [11, 21]])
    check('left gap', solve([[[5, 10], [6, 11]], [[6, 20]]]), [[11, 20]])
    check('right gap', solve([[[6, 10]], [[5, 20], [6, 21]]]), [[10, 21]])
    check('single match', solve([[[5, 10]], [[5, 20]]]), [[10, 20]])
    check('no overlap', solve([[[5, 10]], [[7, 20]]]), [])
    check('empty left', solve([[], [[5, 20]]]), [])
    check('empty right', solve([[[5, 10]], []]), [])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
duplicate runs[[10, 20], [10, 21]][[10, 20], [10, 21], [11, 20], [11, 21]]Failed
left gap[[11, 20]][[11, 20]]Passed
right gap[[10, 21]][[10, 21]]Passed
single match[[10, 20]][[10, 20]]Passed
no overlap[][]Passed
empty left[][]Passed
empty right[][]Passed

SHA-256 / 40baefbf430043d14347a3d1c6fc0f673bcf3c81ed3742c132de0412e00bca1f

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 7 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Sign in to the archive ↗

Verification & scope

Offline stipulated semantics over valid small inputs; no performance, concurrency, or production-engine conformance claim. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:44:17.685378+00:00.

Case digest / 3ed7a188107de8ad533ffcae93680284386e587b53d1fe74d07f16db762f3815