Currently the new reversing sstable algorithms do not support fast
forwarding and the cache does not yet handle reversed results. This
forced us to disable the cache for reversed queries if we want to
guarantee bounded memory. We introduce an option that does this
automatically (without specifying `bypass cache` in the query) and turn
it on by default.
If the user decides that they prefer to keep the cache at the
cost of fetching entire partitions into memory (which may be viable
if their partitions are small) during reversed queries, the option can
be turned off. It is live-updateable.
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