Safely Adding NOT NULL Columns to Your Database Tables - Shopify

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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 based on backward compatibility and data loss prevention. 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 matrix of results is presented, concluding that adding NOT NULL columns without DEFAULT values is risky, and UNIQUE indexes require extreme caution due to potential data loss.