batch_statement::verify_batch_size() verifies that the total size of
mutations generated by the batch statement is smaller than certain
configurable thresholds. This is done by a custom mutation_partition
visitor, which violates atomic_cell_view::value() preconditions by
calling it even for dead cells.
The simples solution is to use
mutation_partition::external_memory_usage() instead.
Message-Id: <20180619131405.12601-1-pdziepak@scylladb.com>
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