Clarify migration comment: nullable-first is about NOT NULL without DEFAULT

Signed-off-by: Fredrik Adelöw <freben@spotify.com> Made-with: Cursor
2026-04-05 14:22:09 +02:00
parent 6941ae10f3
commit 42d0a2749a
1 changed files with 4 additions and 2 deletions
@@ -33,8 +33,10 @@ const MYSQL_BATCH_SIZE = 1000;
 * removed in a future migration) gets an empty string as a placeholder value.
 *
 * The migration adds the column as nullable first, fills every row, then
- * tightens it to NOT NULL. This avoids the MySQL strict-mode restriction that
- * TEXT columns cannot have DEFAULT values.
+ * tightens it to NOT NULL. Adding a NOT NULL column to a non-empty table
+ * requires a DEFAULT so the database can back-fill existing rows; supplying a
+ * sentinel default here would be misleading, so instead we add the column as
+ * nullable, fill every row with its real value, then tighten the constraint.
 *
 * Postgres:  single `UPDATE … FROM unnest(ids::uuid[], refs::text[])` for all
 *            rows — the SQL text is fixed-size and the planner does an index