When purging regular tombstone consult the min_live_timestamp, if available.
This is safe since we don't need to protect dead data from resurrection, as it is already dead.
For shadowable_tombstones, consult the min_memtable_live_row_marker_timestamp,
if available, otherwise fallback to the min_live_timestamp.
If we see in a view table a shadowable tombstone with time T, then in any row where the row marker's timestamp is higher than T the shadowable tombstone is completely ignored and it doesn't hide any data in any column, so the shadowable tombstone can be safely purged without any effect or risk resurrecting any deleted data.
In other words, rows which might cause problems for purging a shadowable tombstone with time T are rows with row markers older or equal T. So to know if a whole sstable can cause problems for shadowable tombstone of time T, we need to check if the sstable's oldest row marker (and not oldest column) is older or equal T. And the same check applies similarly to the memtable.
If both extended timestamp statistics are missing, fallback to the legacy (and inaccurate) min_timestamp.
Fixesscylladb/scylladb#20423Fixesscylladb/scylladb#20424
> [!NOTE]
> no backport needed at this time
> We may consider backport later on after given some soak time in master/enterprise
> since we do see tombstone accumulation in the field under some materialized views workloads
Closesscylladb/scylladb#20446
* github.com:scylladb/scylladb:
cql-pytest: add test_compaction_tombstone_gc
sstable_compaction_test: add mv_tombstone_purge_test
sstable_compaction_test: tombstone_purge_test: test that old deleted data do not inhibit tombstone garbage collection
sstable_compaction_test: tombstone_purge_test: add testlog debugging
sstable_compaction_test: tombstone_purge_test: make_expiring: use next_timestamp
sstable, compaction: add debug logging for extended min timestamp stats
compaction: get_max_purgeable_timestamp: use memtable and sstable extended timestamp stats
compaction: define max_purgeable_fn
tombstone: can_gc_fn: move declaration to compaction_garbage_collector.hh
sstables: scylla_metadata: add ext_timestamp_stats
compaction_group, storage_group, table_state: add extended timestamp stats getters
sstables, memtable: track live timestamps
memtable_encoding_stats_collector: update row_marker: do nothing if missing
c7c5817808