# Skewcheck: ⚠️ ADVISORY — changed behaviour, not proven incompatible

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

Something changed for the previous application version, but the evidence is not sufficient to call it a compatibility regression.

| | |
|---|---|
| Project | fixture/C-assertion-drift |
| Execution | `example-C-assertion-drift` |
| Baseline (old app) | `shop-v1` |
| Candidate | `C-assertion-drift` |
| PostgreSQL | 16.15 (Debian 16.15-1.pgdg12+2) |
| Proposed migration(s) | `002_status_default.sql` |
| Gate | non-blocking (exit 0) |
| Experiment complete | yes |

## Why
- test/orders.test.js: assertion drift

## Recommended next action
Review the listed findings and decide whether the old version's changed behaviour is acceptable during rollout.

## Evidence
### assertion drift — `test/orders.test.js`
**Observed:**
- test/orders.test.js passes on the baseline schema and fails in all candidate runs without any new database error.
- Test output (tail):   error: |- |     Expected values to be strictly equal: |   code: 'ERR_ASSERTION' |   name: 'AssertionError'
**Interpretation:** Likely a changed value or behaviour rather than a schema incompatibility (behavioural changes in this migration: Column orders.status default 'new'::text → 'pending'::text). A human should decide whether the old application's expectation still matters while both versions run.
**Linked change(s):** `column_default_changed:orders.status`

## Schema changes (catalog diff)
| Change | Risk | Exercised by old app | Tests |
|---|---|---|---|
| Column orders.status default 'new'::text → 'pending'::text | behavioral | yes (2 stmt) | `test/orders.test.js` |

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

## 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 | 1/1/0 | 0 | 4 |
| candidate #2 (old app × proposed schema) | 2/2 applied | 1/1/0 | 0 | 4 |

## 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 · 2026-10-24T18:29:57.610Z</sub>