Safely Adding NOT NULL Columns to Your Database Tables - Shopify

This post details an investigation into the safety of adding NOT NULL columns to MySQL tables using the Large Hadron Migrator (LHM) gem. It outlines the LHM shadow-table mechanism and defines schema change safety in terms of backward compatibility and data loss. The investigation focuses on the impact of DEFAULT values, UNIQUE indexes, and MySQL's strict/non-strict modes on the safety of adding NOT NULL columns. A results matrix is presented, concluding that adding NOT NULL columns without a DEFAULT value should be avoided, and caution is advised when adding UNIQUE indexes due to the risk of data loss.