ALTER
Evolve supported table fields, control transforms, and change storage topology.
Table columns
Add one column definition per statement:
ALTER TABLE accounts
ADD COLUMN region LowCardinality(String) DEFAULT 'unknown';
ALTER TABLE accounts
DROP COLUMN legacy_label;Defaults are applied where the engine must materialize an omitted value. Dropping a column is a destructive schema operation: first remove queries, views, and transforms that depend on it, and preserve a recoverable backup.
Attributes
Attribute tables have explicit attribute operations:
ALTER TABLE account_attributes
ADD ATTRIBUTE risk_score UInt64;
ALTER TABLE account_attributes
DROP ATTRIBUTE old_tier;ADD ATTRIBUTE and DROP ATTRIBUTE reject ordinary append or keyed tables.
Transforms
ALTER TRANSFORM copy_trades PAUSE;
ALTER TRANSFORM copy_trades RESUME;See CREATE TRANSFORM for target and restart constraints.
Storage catalog
The experimental topology catalog supports policy changes and controlled resource membership:
ALTER STORAGE POOL fast ADD RESOURCE nvme2;
ALTER STORAGE POOL fast DRAIN RESOURCE nvme1;
ALTER STORAGE POOL fast REMOVE RESOURCE nvme1;Drain and verify a resource before removal. See Storage topology DDL.
The current SQL surface does not provide a general ALTER TABLE ... MODIFY COLUMN migration framework. For incompatible type, key, policy, or index
changes, create a replacement table, copy and validate the data, redirect
dependents, and retire the old object through a controlled migration.