ClickHouse/docs/en/operations/system-tables/detached_parts.md

system.detached_parts

Contains information about detached parts of MergeTree tables. The reason column specifies why the part was detached.

For user-detached parts, the reason is empty. Such parts can be attached with ALTER TABLE ATTACH PARTITION|PART command.

For the description of other columns, see system.parts.

If part name is invalid, values of some columns may be NULL. Such parts can be deleted with ALTER TABLE DROP DETACHED PART.

Original article