Database Migration Planner

Change: add a non-null currency column to orders. Steps (expand then contract): 1) add the column as nullable with a default — no lock on most engines. 2) backfill existing rows in batches to avoid long locks. 3) deploy app code that writes the column. 4) add the not-null constraint once all rows are populated. Rollback: each step reverses cleanly — drop the constraint, then the column. Verify before: row count and a sample. Verify after: no nulls remain, app reads and writes succeed. Locking risk: adding not-null directly would scan the whole table — that is why it is the last step.