How do you run schema migrations without downtime?

Summary

Use the expand/contract pattern: first make backward-compatible additions (new nullable columns, new tables), deploy code that writes to both or reads the new shape, backfill data in batches, switch reads, then remove old columns in a later release.

Longer Answer

Avoid long locks: add indexes concurrently (CREATE INDEX CONCURRENTLY), avoid table rewrites on huge tables (adding a column with a volatile default), set lock timeouts, and test migrations against production-sized data.

Key Points

• Expand → migrate → contract
• Batched backfills
• Concurrent index creation
• Lock timeouts
• Old and new code must coexist