Destructive schema changes: DROP TABLE, DROP COLUMN, TRUNCATE

These statements are not slow — on 8.0.29+ DROP COLUMN is even instant. The risk is different: the data is gone, and MySQL DDL cannot be rolled back. A migration tool's "down" step cannot restore rows it never saved.

What the manual says

• DDL statements cause an implicit commit; they are not part of your transaction and cannot be rolled back after they complete.
• DROP TABLE removes the table definition and all data; TRUNCATE TABLE drops and re-creates the table (it is DDL, not a DELETE).
• Dropping a column: Instant Yes* (8.0.29+), rebuilds table, concurrent DML permitted (Column operations) — fast, and irreversible.

The safe pattern: expand / contract

1. Stop writing the column or table in application code; deploy.
2. Stop reading it; deploy; verify with query logs or metrics that nothing touches it.
3. Only then drop it — in a later migration, with a verified backup if the data has any value.

How Presift flags it

✖ MG001 ERROR  migration.sql:1
    ALTER TABLE users DROP COLUMN legacy_flag permanently discards the column's data
    Why: Dropping a column removes its data for every row; the migration tool cannot restore it. (On MySQL 8.0.29+ DROP COLUMN itself can be INSTANT; the risk here is data loss, not locking.)
    Do:  Prefer expand/contract: stop reading/writing the column in application code first, deploy, verify, then drop in a later migration. If intentional, annotate: -- presift: allow MG001 <reason>

✖ MG001 ERROR  migration.sql:1
    DROP TABLE sessions_old permanently removes the object and its data
    Why: DROP TABLE cannot be rolled back; MySQL DDL is not transactional.

An intentional drop stays visible but no longer fails the check when annotated: -- presift: allow MG001 table unused since 2025-01, backup verified. The reason is recorded in the report, so the review trail lives in the migration itself.

name: migration-safety
on: [pull_request]
permissions:
  contents: read
  id-token: write          # lets Presift start your 30-day evaluation
jobs:
  presift:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: nishu-sde/presift@v1
        with:
          paths: db/migrations
          fail-on: error