# Skewcheck: ❔ NO EVIDENCE — changed objects were not exercised

> **Question:** does the previous application version still work against the proposed database schema?

The experiment ran, but the previous application version's tests did not exercise every risk-bearing change, so compatibility is not established.

| | |
|---|---|
| Project | fixture/D-no-evidence |
| Execution | `example-D-no-evidence` |
| Baseline (old app) | `shop-v1` |
| Candidate | `D-no-evidence` |
| PostgreSQL | 16.15 (Debian 16.15-1.pgdg12+2) |
| Proposed migration(s) | `002_drop_legacy_notes.sql` |
| Gate | **blocking** (exit 3) |
| Experiment complete | yes |

## Why
- Untested risk-bearing change: customers.legacy_notes (column_removed).

## Recommended next action
Add old-application tests that exercise the untested changes, or have a human reviewer explicitly accept the risk.

## Schema changes (catalog diff)
| Change | Risk | Exercised by old app | Tests |
|---|---|---|---|
| Column customers.legacy_notes (text) removed | breaking | **no** | — |

**Coverage:** 0 of 1 risk-bearing schema change(s) were exercised by the old application's tests in the control run.

- Column customers.legacy_notes (text) removed — no statement from the old application's tests touched customers.legacy_notes in the control run.
- Baseline source still mentions it at: src/admin.js:5

## Runs
| Run | Migrations | Tests (pass/fail/other) | DB errors | Statements |
|---|---|---|---|---|
| control (old app × baseline schema) | 1/1 applied | 2/0/0 | 0 | 7 |
| candidate #1 (old app × proposed schema) | 2/2 applied | 2/0/0 | 0 | 7 |
| candidate #2 (old app × proposed schema) | 2/2 applied | 2/0/0 | 0 | 7 |

## Skipped checks
- Current application × proposed schema (no current app configured).
- Performance / lock-time analysis (out of scope).

## Limitations
- Skewcheck only observes code paths that the old application's tests execute; untested paths are reported as coverage gaps, not as safe.
- Coverage is identifier-level: a statement "touches" a changed object when it names the table (and column, or uses *). It is not a SQL parser.
- Correlation between an error and a schema change uses a fixed set of deterministic rules (R1–R6). Anything else is reported as ADVISORY, never UNSAFE.
- Performance, locking and migration runtime on production-sized data are not evaluated.
- Test files of one run share a database and run sequentially; ordering effects are identical across runs but are not isolated.
- Data-only migration effects are detected only when a test asserts on the affected data.

<sub>skewcheck 0.1.0 · report schema v1 · 2026-10-24T18:29:58.997Z</sub>