  While many researchers adopt a sharding approach to design scaling
blockchains, few works have studied the transaction placement problem incurred
by sharding protocols. The widely-used hashing placement algorithm renders an
overwhelming portion of transactions as cross-shard. In this paper, we analyze
the high cost of cross-shard transactions and reveal that most Bitcoin
transactions have simple dependencies and can become single-shard under a
placement algorithm taking transaction dependencies into account. In addition,
we perform a case study of OptChain, which is the state-of-the-art transaction
placement algorithm for sharded blockchains, and find a shortcoming of it. A
simple fix is proposed, and our evaluation results demonstrate that the
proposed fix effectively helps OptChain overcome the shortcoming and
significantly improve the system performance under a special workload. The
authors of OptChain made some revisions to the algorithm description after
noticing our work. Their updated algorithm does not exhibit the shortcoming
under the workloads employed by this paper.
