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database-migrations

Skill0 starsChanged 26 days agoMITView on GitHub

Safe schema and data migrations: ordering, immutability, locks, backfills, persisted enums and rollback. Trigger: When creating or editing files under migrations/, db/migrate or any .sql file.

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SKILL.md

Database migrations

Activation

Apply to any file under migrations/ or db/migrate/ and to .sql files, in any language or ORM. Migration paths are high risk; do not lower it.

Inputs

  • The migration tool and its ordering/checksum rules.
  • Target engine and version; table size and traffic when known.
  • Code that reads or writes the changed columns.

Procedure

  1. Immutability. Condition: a migration was already applied anywhere shared. Action: add a new migration; never edit the old one. Exception: unmerged local migration. Verify: git diff shows only new files in migrations/.
  2. Unique on existing data (migration.unique-existing-data). Condition: adding a unique index or constraint. Action: first query for duplicates, deduplicate or fail with a clear message, then add the constraint. Exception: table is provably empty in every environment. Verify: a test migrating a fixture with duplicates.
  3. Persisted enums (data.persisted-enum). Condition: an enum is stored. Action: store a stable code, not an ordinal or a name that may be renamed; additions are additive, removals need a data migration. Exception: none for stored values. Verify: a test reading a row written by the previous version.
  4. Locks and backfills. Condition: large table. Action: split into add nullable column, batched backfill, then constraint; use the engine's non-blocking index option where available. Exception: small tables. Verify: state the lock taken and the expected duration in the Evidence.
  5. Rollback. Condition: every migration. Action: write the down step or state that the change is forward-only and why. Verify: apply, roll back (or restore) and re-apply on a scratch database.
  6. Code compatibility. Condition: a column is dropped or renamed. Action: deploy code that stops using it first (expand/contract). Verify: grep shows no remaining reader.

Decisions and exceptions

  • Mixing schema change and large data change in one migration: split.
  • Destructive statements (DROP, TRUNCATE) need explicit Task Card approval.

Evidence

The command that applied the migration on a scratch database, its exit code, and the rollback result. Without a database, report the migration as unrun.

References

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