GitLab got bit: MySQL fails at Rails migrations that work in PostgreSQL

This post highlights a critical issue encountered during a database migration for GitLab 7.6, specifically a failure in MySQL when removing columns that had a composite index. It details how a seemingly simple migration that worked in PostgreSQL failed in MySQL due to the database's behavior in rebuilding indexes. The post explains the fix by adding a check for index existence before removal and emphasizes the importance of thorough testing across different database backends during the update process.