When garbage collecting tombstones, we care only
about shadowing of live data. However, currently
we track min/max timestamp of both live and dead
data, but there is no problem with purging tombstones
that shadow dead data (expired or shdowed by other
tombstones in the sstable/memtable).
Also, for shadowable tombstones, we track live row marker timestamps
separately since, if the live row marker timestamp is greater than
a shadowable tombstone timestamp, then the row marker
would shadow the shadowable tombstone thus exposing the cells
in that row, even if their timestasmp may be smaller
than the shadow tombstone's.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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