Currently, when two cells have the same write timestamp
and both are alive or expiring, we compare their value first,
before checking if either of them is expiring
and if both are expiring, comparing their expiration time
and ttl value to determine which of them will expire
later or was written later.
This was changed in CASSANDRA-14592
for consistency with the preference for dead cells over live cells,
as expiring cells will become tombstones at a future time
and then they'd win over live cells with the same timestamp,
hence they should win also before expiration.
In addition, comparing the cell value before expiration
can lead to unintuitive corner cases where rewriting
a cell using the same timestamp but different TTL
may cause scylla to return the cell with null value
if it expired in the meanwhile.
Also, when multiple columns are written using two upserts
using the same write timestamp but with different expiration,
selecting cells by their value may return a mixed result
where each cell is selected individually from either upsert,
by picking the cells with the largest values for each column,
while using the expiration time to break tie will lead
to a more consistent results where a set of cell from
only one of the upserts will be selected.
Fixesscylladb/scylladb#14182
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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