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`system.view_building_tasks` is a single-partition Raft group0 table (pk = `"view_building"`, CK = timeuuid). When `clean_finished_tasks()` deletes hundreds of finished tasks, the physical rows remain in SSTables until compaction. Any subsequent read of the partition counts every column of every tombstoned row as a dead cell, triggering `tombstone_warn_threshold` warnings in large clusters. Two-part fix: **1. Range tombstones instead of row tombstones (commits 2–3)** Instead of one row tombstone per finished task, find the minimum alive task UUID (`min_alive_uuid`) and emit a single range tombstone `[before_all, min_alive_uuid)` covering all tasks below that boundary. This reduces the tombstone count significantly and also benefits future compaction. **2. Bounded scan with `min_task_id` (commits 4–6)** Even with range tombstones, physical rows remain until compaction and still count as dead cells during reads. The only way to avoid them is to not read them at all. - Add a `min_task_id timeuuid` static column to `system.view_building_tasks`. - On every GC, write `min_task_id = min_alive_uuid` atomically with the range tombstone (same Raft batch). - On reload, read `min_task_id` first using a **static-only partition slice** (empty `_row_ranges` + `always_return_static_content`): the SSTable reader stops immediately after the static row before processing any clustering tombstones — zero dead cells counted. - Use `AND id >= min_task_id` as a lower bound for the main task scan, skipping all tombstoned rows. The static-only read and the bounded scan are gated on the `VIEW_BUILDING_TASKS_MIN_TASK_ID` cluster feature so mixed-version clusters fall back to the full scan. The issue is not critical, so the fix shouldn't be backported. Fixes SCYLLADB-657 Closes scylladb/scylladb#28929 * github.com:scylladb/scylladb: test/cluster/test_view_building_coordinator: add reproducer for tombstone threshold warning docs: document tombstone avoidance in view_building_tasks view_building: add `task_uuid_generator` to `view_building_task_mutation_builder` view_building: introduce `task_uuid_generator` view_building: store `min_alive_uuid` in view building state view_building: set min_task_id when GC-ing finished tasks view_building: add min_task_id support to view_building_task_mutation_builder view_building: add min_task_id static column and bounded scan to system_keyspace view_building: use range tombstone when GC-ing finished tasks view_building: add range tombstone support to view_building_task_mutation_builder view_building: introduce VIEW_BUILDING_TASKS_MIN_TASK_ID cluster feature