Migration Patterns

ALTER TABLE Operations

• Add column: ALTER TABLE t ADD COLUMN col Type [DEFAULT expr] [AFTER existing_col]
• Drop column: ALTER TABLE t DROP COLUMN col
• Modify type: ALTER TABLE t MODIFY COLUMN col NewType (must be compatible)
• Rename: ALTER TABLE t RENAME COLUMN old TO new
• These are metadata-only operations — instant for most changes

Engine Changes

• Cannot ALTER engine directly
• Pattern: create new table → insert from old → rename

CREATE TABLE t_new ENGINE = ReplacingMergeTree() ORDER BY id AS SELECT * FROM t_old;
RENAME TABLE t_old TO t_backup, t_new TO t_old;

• For large tables: use INSERT INTO ... SELECT with batching