Github user thobbs commented on the pull request:
https://github.com/apache/cassandra/commit/55f7a53e44b5d9af767464872b74a721717ba216#commitcomment-17355539
In src/java/org/apache/cassandra/cql3/Maps.java:
In src/java/org/apache/cassandra/cql3/Maps.java on line 399:
Setting on an arbitrary level shouldn't fundamentally require a read before
write. However, it occurred to me that in the 3.x storage format, we don't
have a tombstone that can delete a slice of cells within a collection --
ComplexColumnDeletions can only cover a full column. This may be something
that we need to implement for good nested collection support, but would have to
wait until 4.0 for the storage format change.
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