In process of doing my second major data purge from a cassandra system. Almost all of my purging is done via row tombstones. While performing this the second time while trying to cajole compaction to occur (in 2.1.x, LevelledCompaction) to goddamn actually compact the data, I've been thinking as to why there isn't a separate set of sstable infrastructure setup for row deletion tombstones.
I'm imagining that row tombstones are written to separate sstables than mainline data updates/appends and range/column tombstones. By writing them to separate sstables, the compaction systems can preferentially merge / process them when compacting sstables. This would create an additional sstable for lookup in the bloom filters, granted. I had visions of short circuiting the lookups to other sstables if a row tombstone was present in one of the special row tombstone sstables. But that would only be possible if there was the notion of a "super row tombstone" that permanently deleted a rowkey and all future writes would be invalidated. Kind of like how a tombstone with a mistakenly huge timestamp becomes a sneaky permanent tombstone, but intended. There could be a special operation / statement to undo this permanent tombstone, and since the row tombstones would be in their own dedicated sstables, they could process and compact more quickly, with prioritization by the compactor. I'm thinking there must be something I am forgetting in the read/write/compaction paths that invalidate this.