Eight tracks

Each one is prose with a live database beside it. Every claim a lesson makes is a statement you can run against a forty-one-million-row table with a workload going through it.

• Locks

  0/2

  What is waiting behind your migration?

  The eight lock levels and what conflicts with what, the queue they wait in, and head-of-line blocking watched as it happens. Ends with `lock_timeout`, which belongs at the top of every migration you will ever write.

  • Eight levels, and what conflicts with what9 min
  • Head-of-line blocking10 min

• Rewrite or metadata

  0/2

  Is this statement copying every row?

  Constant defaults against volatile ones, type changes, `NOT NULL` and its two-step form, and what a full-table rewrite does to replication lag. The single most valuable distinction in the whole subject.

  • The default that rewrites, and the one everybody warns you about11 min
  • Which type changes are free10 min

• Indexes

  0/1

  How do you build one without stopping writes?

  `CONCURRENTLY` and what it costs, invalid indexes and how to clean them up, unique indexes on live data, and dropping the index nobody uses.

  • Building an index without stopping writes9 min

• Constraints

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  How do you add a rule to a table already full of rows?

  `NOT VALID` then `VALIDATE`, foreign keys on large tables and the lock they take on both sides, and check constraints as the cheap alternative to a type change.

  • NOT VALID, then VALIDATE10 min

• Expand and contract

  0/1

  How do you change a column the running code still reads?

  Adding beside, dual writes, backfill, switch, drop — and why a rename is five migrations rather than one. Views as a shim when the old name has to keep working.

  • Why a rename is five migrations11 min

• Backfills

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  How do you touch ten million rows without anybody noticing?

  Batch size and what decides it, sleeping between batches, writing a backfill you can safely run twice, and watching p99 while it runs.

  • Ten million rows, and nobody notices10 min

• Schemaless drift

  0/1

  Where does the schema go when there isn't one?

  Versioned documents, keeping a reader for every version still in the store, backfill as a batch job, and deciding when a version is finally safe to stop supporting.

  • Where the schema goes when there isn't one11 min

• Reading a generated migration

  Lands in a later milestone

  What will this do to production?

  The track the rest exists for. You are handed the migration file an agent wrote for a stated change and a production-sized table, and asked what it locks, for how long, whether it rewrites, and what the old code does mid-migration. Then you run it and watch.

  It lives at /audit

9 lessons built so far. The rest land with their milestones.