# Cross-DBMS verification: PostgreSQL 16.15

Status: the evaluation mechanism was re-run on PostgreSQL for the 12 seed families on
2026-10-19. Full-corpus PostgreSQL evidence is not published in v0.2.0; this document
records the portability check and its limits.

## What was verified

The published MySQL evidence for the 12 seed families of the library schema was
recomputed on PostgreSQL 16.15 from the same inputs:

| Input | Value |
|---|---|
| Corpus | `data/corpus/families.json` (12 families, unchanged reference SQL) |
| State bank | `data/states/states.json`, the same 28 states |
| Model answers | the saved runs in `data/runs/` (three seed models, unchanged) |
| DBMS | PostgreSQL 16.15 (previous series preflight used the same version) |
| DDL | the corpus DDL, executed without any adaptation |
| Reference executions | 12 families × 28 states = 336 |
| Candidate checks | 12 families × 28 states × 3 models = 1 008 |

For every state the reference and the candidate were executed read-only in a clean
private schema, and the resulting row lists were compared; the per-(model, family, state)
outcome was then compared with the refuting-state sets published in `data/evidence/`.

## Result

- **DDL adaptation required: none.** The library DDL (`INTEGER PRIMARY KEY`,
  `VARCHAR` lengths, inline foreign keys) and all 12 reference queries executed on
  PostgreSQL unchanged.
- **Status mismatches with the MySQL evidence: 0** of 1 008 candidate checks.

The raw check record is stored in [cross_dbms_seed_check.json](cross_dbms_seed_check.json).
No per-state rows or pins are duplicated there; the record is a verification log, not a
second evidence bank.

## Why the comparison is meaningful, and what it does not show

- Equivalence is checked **within one engine**: the reference and the candidate run on the
  same DBMS, so engine-specific row order rules do not affect the verdict. MySQL sorts
  NULLs first in ascending order and PostgreSQL sorts them last, so the *ordering of rows
  in a witness* can differ between engines; the refuting state and the multiset difference
  do not.
- The check covers the 12 seed families only. It shows that the mechanism ports, not that
  every family in the corpus behaves identically on PostgreSQL. In particular, queries
  with engine-specific functions would need dialect review before a full PostgreSQL run.
- The dataset format stores one `dbms` per evidence file. Adding a second engine as
  first-class evidence requires a format extension (separate evidence per engine or a
  `dbms` field per record) and is left for the next version.