Summary
Tests set dialect: 'mysql'/'postgres' only to assert generated SQL text (migrations, foreign-keys-inline, types-and-inference). No test ever executes CRUD/relations against a real MySQL/Postgres, so the entire DriverExecutor class (RETURNING insert, LAST_INSERT_ID() fallback, extractChanges/extractInsertId against real driver result shapes) and the dozens of config.dialect === 'postgres'/'mysql' runtime branches in client.ts are unverified.
This is the root cause that lets dialect bugs slip through (see the batch of placeholder/JSON/union/change-count issues filed alongside this).
Suggested fix
Add a containerized integration suite / CI matrix that actually runs CRUD + relations against pg + mysql. Even a smoke test of DriverExecutor.insert/run would catch result-shape regressions.
Severity: high (process gap). Found via internal code audit.