https://vacua.dev/tags/migrations/Recent content in Migrations on Brian FaustHugoen-USMon, 01 Jan 0001 00:00:00 +0000https://vacua.dev/thoughts/a-migration-is-a-production-change/Wed, 01 Mar 2017 00:00:00 +0000https://vacua.dev/thoughts/a-migration-is-a-production-change/<p>We deployed an additive schema change which the database could execute without rewriting the table. It still waited indefinitely.</p> <p>The statement needed an exclusive table lock. A long-running session already held a conflicting lock, so the migration queued. New application traffic then queued behind the migration. The column addition was cheap after lock acquisition; waiting to begin was the incident.</p> <p>Another schema change looked cheap for the opposite reason: remove fields and tables which the application no longer read. The SQL was small. The decision would erase financial history from data sets measured in tens of gigabytes.</p>