The following operations with projections are available:
ALTER TABLE [db].name ADD PROJECTION name AS SELECT <COLUMN LIST EXPR> [GROUP BY] [ORDER BY]- Adds projection description to tables metadata.
ALTER TABLE [db].name DROP PROJECTION name- Removes projection description from tables metadata and deletes projection files from disk.
ALTER TABLE [db.]table MATERIALIZE PROJECTION name IN PARTITION partition_name- The query rebuilds the projection
namein the partition
partition_name. Implemented as a mutation.
ALTER TABLE [db.]table CLEAR PROJECTION name IN PARTITION partition_name- Deletes projection files from disk without removing description.
CLEAR are lightweight in a sense that they only change metadata or remove files.
Also, they are replicated, syncing projections metadata via ZooKeeper.